Systems and methods for electrode assembly for redox flow battery system

ABSTRACT

Systems and methods are provided for assembling and operating an electrode assembly for a redox flow battery system. In one example, the electrode assembly may include an inflatable housing in which a negative electrode spacer and a positive electrode may be positioned, wherein the inflatable housing may inflate responsive to applied internal pressure during operation of the redox flow battery system. In some examples, the electrode assembly may be assembled via roll-to-roll processing and may be mechanically and fluidically coupled to electrode assemblies of like configuration. In this way, tolerance stacking may be decreased, processing may be simplified, and costs may be reduced relative to molding-based processes for electrode assembly manufacturing.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/655,492 entitled “SYSTEMS AND METHODS FOR ELECTRODE ASSEMBLYFOR REDOX FLOW BATTERY SYSTEM” filed on Mar. 18, 2022. U.S. patentapplication Ser. No. 17/655,492 claims priority to U.S. ProvisionalApplication No. 63/187,358 entitled “SYSTEMS AND METHODS FOR ELECTRODEASSEMBLY FOR REDOX FLOW BATTERY SYSTEM” and filed on May 11, 2021. Theentire contents of each of the above-identified applications are herebyincorporated by reference for all purposes.

FIELD

The present description relates generally to systems and methods forassembling and operating an electrode assembly, particularly for a redoxflow battery system.

BACKGROUND AND SUMMARY

Redox flow batteries are suitable for grid-scale storage applicationsdue to their capability for scaling power and capacity independently, aswell as for charging and discharging over thousands of cycles withreduced performance losses in comparison to conventional batterytechnologies. An all-iron hybrid redox flow battery is particularlyattractive due to incorporation of low-cost, earth-abundant materials.In general, iron redox flow batteries (IFBs) rely on iron, salt, andwater for electrolyte, thus including simple, earth-abundant, andinexpensive materials, and eliminating incorporation of harsh chemicalsand reducing an environmental footprint thereof.

A given IFB may include a flow cell having a positive (redox) electrodewhere a redox reaction occurs and a negative (plating) electrode whereferrous iron (Fe²⁺) in the electrolyte may be reduced and plated via aplating reaction. The electrolyte transporting ions for the redox andplating reactions may be pumped through compartments housing thepositive and negative electrodes to cycle ions for the redox and platingreactions, as well as through rebalancing cells to reduce excess ferriciron (Fe₃ ⁺) and rebalance the electrolyte for subsequent pumping backinto the compartments. In some examples, the IFB may include a pluralityof flow cells coupled in series, often employing complex fluiddistribution systems to pump and cycle the electrolyte through the IFBs.

In large format IFB s, correspondingly large molding tools may be usedto manufacture assembly components. Such molding tools may be expensive,specialized, and difficult to modify or adjust if fine tuning of theassembly components is desired. Similarly, large and expensive handlingequipment may be employed to move the assembly components alongproduction lines. Accordingly, excessive floor space may be occupied bythe molding tools and handling equipment alone. Additionally, when suchlarge format IFB s include many flow cells coupled in series, hundredsof components may be stacked, resulting in tolerance stacking issues.Because of the difficulty in modifying the molding tools, such tolerancestacking issues may be practically unavoidable with a molding setup.

In one example, the issues described above may be addressed by anelectrode assembly for a redox flow battery, the electrode assemblyincluding an inflatable housing, the inflatable housing at leastpartially enclosing an internal volume, the internal volume includingnegative and positive electrode compartments, a negative electrodespacer positioned in the negative electrode compartment, and a positiveelectrode positioned in the positive electrode compartment, wherein theinflatable housing may inflate responsive to applied internal pressureto increase the internal volume of the electrode assembly duringoperation of the redox flow battery. In some examples, the electrodeassembly may be included in a stack of electrode assemblies, whereinelectrode assemblies are fluidically coupled to one another via commonfluid manifolds formed from aligning electrolyte ports disposed in theinflatable housing with electrolyte ports of adjacent electrolyteassemblies. In one example, each of the electrolyte ports may include anelectrolyte distribution insert, which may interlock with correspondingelectrolyte distribution inserts included in the electrolyte ports ofthe adjacent electrolyte assemblies and seal the common fluid manifold.Moreover, in some examples, the electrode assembly may be manufacturedvia roll-to-roll processing, wherein extruded thermoplastic sheets maybe welded to one another and cut from a roll to form the inflatablehousing. In such examples, additional components of the electrodeassembly may be welded to the extruded thermoplastic sheets or disposedin cavities or compartments formed during such roll-to-roll processing.In this way, a structurally stable and flexible electrode assembly maybe formed in which tolerance stacking may be reduced relative tomolding-based electrode assembly manufacturing processes.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an example redox flow battery systemincluding a membrane electrode assembly (MEA) with redox and platingelectrodes.

FIG. 2A shows an exploded view of an MEA stack.

FIG. 2B shows an exploded view of an MEA of the MEA stack of FIG. 2A.

FIG. 3 shows a schematic cross-sectional view of the MEA of FIG. 2B.

FIGS. 4A and 4B show perspective cross-sectional views of electrolyteinserts of the MEA of FIG. 2B.

FIG. 4C shows a schematic cross-sectional view of one of the electrolyteinserts of the MEA of FIG. 2B.

FIGS. 5A and 5B show perspective cross-sectional views of electrolytepassage configurations of the MEA of FIG. 2B.

FIG. 5C shows a schematic top view of electrolyte passage configurationsof the MEA of FIG. 2B.

FIG. 6 shows a flow chart of a method for assembling an MEA stack viaroll-to-roll processing, operating the MEA stack as a redox flowbattery, and testing the MEA stack.

FIG. 7 shows a flow chart of a method for assembling an MEA viaroll-to-roll processing.

FIG. 8 shows a schematic diagram of an exemplary roll-to-roll processingconfiguration for assembling an MEA stack.

DETAILED DESCRIPTION

The following description relates to systems and methods for assemblinga membrane electrode assembly (MEA) via roll-to-roll processing andoperating the MEA as a redox flow battery. As used herein, “membraneelectrode assembly” or “MEA” may refer to a battery configurationincluding a housing with an integrated membrane delimiting electrodecompartments, chambers, or cavities. In an exemplary embodiment, the MEAmay be included in an MEA stack of a redox flow battery system andfluidically coupled to an electrolyte subsystem via at least one commonfluid manifold. The redox flow battery system is depicted schematicallyin FIG. 1 , with the MEA fluidically coupled to an integratedmulti-chambered tank having separate positive and negative electrolytechambers. In some examples, the redox flow battery system may be anall-iron flow battery (IFB) utilizing iron redox chemistry at both apositive (redox) electrode and the negative (plating) electrode of theIFB. The electrolyte chambers may be coupled to one or more batterycells, each cell including the positive and negative electrodes.Therefrom, electrolyte may be pumped through positive and negativeelectrode compartments respectively housing the positive and negativeelectrodes.

In some examples, the redox flow battery system may be a hybrid redoxflow battery system. Hybrid redox flow batteries are redox flowbatteries which may be characterized by deposition of one or moreelectroactive materials as a solid layer on an electrode (e.g., thenegative electrode). Hybrid redox flow batteries may, for instance,include a chemical species which may plate via an electrochemicalreaction as a solid on a substrate throughout a battery charge process.During battery discharge, the plated species may ionize via a furtherelectrochemical reaction, becoming soluble in the electrolyte. In hybridredox flow battery systems, a charge capacity (e.g., a maximum amount ofenergy stored) of the redox flow battery may be limited by an amount ofmetal plated during battery charge and may accordingly depend on anefficiency of the plating system as well as volume and surface areaavailable for plating.

However the redox flow battery system is configured, whether as an IFB,a hybrid redox flow battery system, or a combination thereof (e.g., anall-iron hybrid redox flow battery system), in some examples, many redoxflow battery cells may be arranged in series to achieve a desiredperformance and output. Such arrangements may result in undesirably hightolerance stacking, which may be problematic in assembly, operation, andservicing of the redox flow battery system. To reduce such tolerancestacking, and as described by embodiments herein, the redox flow batterysystem may include an MEA stack, such as the MEA stack of FIG. 2A,including a plurality of MEAs, such as the MEA depicted in the explodedview of FIG. 2B and the schematic cross-section of FIG. 3 , assembledvia roll-to-roll processing of extruded thermoplastics. Exemplarymethods for assembling the MEA stack via such roll-to-roll processing,operating the MEA stack, and testing the MEA stack are provided in FIGS.6 and 7 . One exemplary roll-to-roll processing configuration forassembling the MEA stack is schematically provided in FIG. 8 .

When MEAs in the MEA stack are operated as redox flow battery cells ofthe redox flow battery system, positive and negative electrolytes may becycled therethrough to replenish and replace ions for charging ordischarging of the redox flow battery system. Accordingly, the MEAs mayinclude electrolyte ports fluidically coupled to common fluid manifoldswhich span the MEA stack and which are fluidically coupled, in turn, tothe electrolyte subsystem to permit cycling of the positive and negativeelectrolytes. In some examples, and as shown in FIGS. 4A-4C, theelectrolyte ports of a given MEA may respectively include electrolytedistribution inserts which may mechanically interlock with electrolytedistribution inserts of adjacent MEAs to fluidically couple the MEAs ofthe MEA stack and form the common fluid manifolds (as used herein,“adjacent” may describe any two components having no interveningcomponents therebetween). The positive and negative electrolytes mayenter electrolyte ports of each MEA of the MEA stack via channels in theelectrolyte distribution inserts, flowing therefrom to respectivepositive and negative electrode compartments via respective positive andnegative electrolyte passages. One exemplary configuration of thepositive and negative electrolyte passages is depicted in FIGS. 5A-5C,where the positive and negative electrolyte passages are shown as formedbetween the extruded thermoplastics of the MEA.

As shown in FIG. 1 , in a redox flow battery system 10, a negativeelectrode 26 may be referred to as a plating electrode and a positiveelectrode 28 may be referred to as a redox electrode. A negativeelectrolyte within a plating side (e.g., a negative electrodecompartment 20) of a redox flow battery cell 18 may be referred to as aplating electrolyte, and a positive electrolyte on a redox side (e.g., apositive electrode compartment 22) of the redox flow battery cell 18 maybe referred to as a redox electrolyte.

“Anode” refers to an electrode where electroactive material loseselectrons and “cathode” refers to an electrode where electroactivematerial gains electrons. During battery charge, the negativeelectrolyte gains electrons at the negative electrode 26, and thenegative electrode 26 is the cathode of the electrochemical reaction.During battery discharge, the negative electrolyte loses electrons, andthe negative electrode 26 is the anode of the electrochemical reaction.Alternatively, during battery discharge, the negative electrolyte andthe negative electrode 26 may be respectively referred to as an anolyteand the anode of the electrochemical reaction, while the positiveelectrolyte and the positive electrode 28 may be respectively referredto as a catholyte and the cathode of the electrochemical reaction.During battery charge, the negative electrolyte and the negativeelectrode 26 may be respectively referred to as the catholyte and thecathode of the electrochemical reaction, while the positive electrolyteand the positive electrode 28 may be respectively referred to as theanolyte and the anode of the electrochemical reaction. For simplicity,the terms “positive” and “negative” are used herein to refer to theelectrodes, electrolytes, and electrode compartments in redox flowbattery systems.

One example of a hybrid redox flow battery is an all-iron redox flowbattery (IFB), in which the electrolyte includes iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode 26 includes metal iron. For example, at the negative electrode26, ferrous iron (Fe²⁺) gains two electrons and plates as iron metal(Fe⁰) onto the negative electrode 26 during battery charge, and Fe⁰loses two electrons and re-dissolves as Fe²⁺ during battery discharge.At the positive electrode 28, Fe²⁺ loses an electron to form ferric iron(Fe³⁺) during battery charge, and Fe³⁺ gains an electron to form Fe²⁺during battery discharge. The electrochemical reaction is summarized inequations (1) and (2), wherein the forward reactions (left to right)indicate electrochemical reactions during battery charge, while thereverse reactions (right to left) indicate electrochemical reactionsduring battery discharge:

Fe²⁺+2e ⁻↔Fe⁰−0.44V(negative electrode)  (1)

Fe²⁺↔2Fe³⁺+2e ⁻+0.77V(positive electrode)  (2)

As discussed above, the negative electrolyte used in the IFB may providea sufficient amount of Fe²⁺ so that, during battery charge, Fe²⁺ mayaccept two electrons from the negative electrode 26 to form Fe⁰ andplate onto a substrate. During battery discharge, the plated Fe⁰ maylose two electrons, ionizing into Fe²⁺ and dissolving back into theelectrolyte. An equilibrium potential of the above reaction is −0.44 Vand this reaction therefore provides a negative terminal for the desiredsystem. On the positive side of the IFB, the electrolyte may provideFe²⁺ during battery charge which loses an electron and oxidizes to Fe³⁺.During battery discharge, Fe³⁺ provided by the electrolyte becomes Fe²⁺by absorbing an electron provided by the positive electrode 28. Anequilibrium potential of this reaction is +0.77 V, creating a positiveterminal for the desired system.

The IFB may provide the ability to charge and recharge electrolytestherein in contrast to other battery types utilizing non-regeneratingelectrolytes. Charge may be achieved by respectively applying anelectric current across the electrodes 26 and 28 via terminals 40 and42. The negative electrode 26 may be electrically coupled via theterminal 40 to a negative side of a voltage source so that electrons maybe delivered to the negative electrolyte via the positive electrode 28(e.g., as Fe²⁺ is oxidized to Fe³⁺ in the positive electrolyte in thepositive electrode compartment 22). The electrons provided to thenegative electrode 26 may reduce the Fe²⁺ in the negative electrolyte toform Fe⁰ at the (plating) substrate, causing the Fe²⁺ to plate onto thenegative electrode 26.

Discharge may be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe³⁺ remains available in thepositive electrolyte for reduction. As an example, Fe³⁺ availability maybe maintained by increasing a concentration or a volume of the positiveelectrolyte in the positive electrode compartment 22 side of the redoxflow battery cell 18 to provide additional Fe³⁺ ions via an externalsource, such as an external positive electrolyte chamber 52. Morecommonly, availability of Fe⁰ during discharge may be an issue in IFBsystems, wherein the Fe⁰ available for discharge may be proportional toa surface area and a volume of the negative electrode substrate, as wellas to a plating efficiency. Charge capacity may be dependent on theavailability of Fe²⁺ in the negative electrode compartment 20. As anexample, Fe²⁺ availability may be maintained by providing additionalFe²⁺ ions via an external source, such as an external negativeelectrolyte chamber 50 to increase a concentration or a volume of thenegative electrolyte to the negative electrode compartment 20 side ofthe redox flow battery cell 18.

In an IFB, the positive electrolyte may include ferrous iron, ferriciron, ferric complexes, or any combination thereof, while the negativeelectrolyte may include ferrous iron or ferrous complexes, depending ona state of charge (SOC) of the IFB system. As previously mentioned,utilization of iron ions in both the negative electrolyte and thepositive electrolyte may allow for utilization of the same electrolyticspecies on both sides of the redox flow battery cell 18, which mayreduce electrolyte cross-contamination and may increase the efficiencyof the IFB system, resulting in less electrolyte replacement as comparedto other redox flow battery systems.

Efficiency losses in an IFB may result from electrolyte crossoverthrough a separator 24 (e.g., ion-exchange membrane barrier, microporousmembrane, and the like). For example, Fe³⁺ ions in the positiveelectrolyte may be driven toward the negative electrolyte by a Fe³⁺ ionconcentration gradient and an electrophoretic force across the separator24. Subsequently, Fe³⁺ ions penetrating the separator 24 and crossingover to the negative electrode compartment 20 may result in coulombicefficiency losses. Fe³⁺ ions crossing over from the low pH redox side(e.g., more acidic positive electrode compartment 22) to high pH platingside (e.g., less acidic negative electrode compartment 20) may result inprecipitation of Fe(OH)₃. Precipitation of Fe(OH)₃ may degrade theseparator 24 and cause permanent battery performance and efficiencylosses. For example, Fe(OH)₃ precipitate may chemically foul an organicfunctional group of an ion-exchange membrane or physically clogmicropores of the ion-exchange membrane. In either case, due to theFe(OH)₃ precipitate, membrane ohmic resistance may rise over time andbattery performance may degrade. Precipitate may be removed by washingthe IFB with acid, but constant maintenance and downtime may bedisadvantageous for commercial battery applications. Furthermore,washing may be dependent on regular preparation of electrolyte,contributing to additional processing costs and complexity.Alternatively, adding specific organic acids to the positive electrolyteand the negative electrolyte in response to electrolyte pH changes maymitigate precipitate formation during battery charge and dischargecycling without driving up overall costs. Additionally, implementing amembrane barrier that inhibits Fe³⁺ ion crossover may also mitigatefouling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ gas, and a reaction ofprotons in the negative electrode compartment 20 with electrons suppliedat the plated iron metal of the negative electrode 26 to form H₂ gas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)may be readily available and may be produced at low costs. In oneexample, the IFB electrolyte may be formed from ferrous chloride(FeCl₂), potassium chloride (KCl), manganese(II) chloride (MnCl₂), andboric acid (H₃BO₃). The IFB electrolyte may offer higher reclamationvalue because the same electrolyte may be used for the negativeelectrolyte and the positive electrolyte, consequently reducingcross-contamination issues as compared to other systems. Furthermore,because of iron's electron configuration, iron may solidify into agenerally uniform solid structure during plating thereof on the negativeelectrode substrate. For zinc and other metals commonly used in hybridredox batteries, solid dendritic structures may form during plating. Astable electrode morphology of the IFB system may increase theefficiency of the battery in comparison to other redox flow batteries.Further still, iron redox flow batteries may reduce the use of toxic rawmaterials and may operate at a relatively neutral pH as compared toother redox flow battery electrolytes. Accordingly, IFB systems mayreduce environmental hazards as compared with all other current advancedredox flow battery systems in production.

Continuing with FIG. 1 , a schematic illustration of the redox flowbattery system 10 is shown. The redox flow battery system 10 may includethe redox flow battery cell 18 fluidly coupled to an integratedmulti-chambered electrolyte storage tank 110. The redox flow batterycell 18 may include the negative electrode compartment 20, separator 24,and positive electrode compartment 22. The separator 24 may include anelectrically insulating ionic conducting barrier which prevents bulkmixing of the positive electrolyte and the negative electrolyte whileallowing conductance of specific ions therethrough. For example, and asdiscussed above, the separator 24 may include an ion-exchange membraneand/or a microporous membrane.

The negative electrode compartment 20 may include the negative electrode26, and the negative electrolyte may include electroactive materials.The positive electrode compartment 22 may include the positive electrode28, and the positive electrolyte may include electroactive materials. Insome examples, multiple redox flow battery cells 18 may be combined inseries or in parallel to generate a higher voltage or current in theredox flow battery system 10.

Further illustrated in FIG. 1 are negative and positive electrolytepumps 30 and 32, both used to pump electrolyte solution through theredox flow battery system 10. Electrolytes are stored in one or moretanks external to the cell, and are pumped via the negative and positiveelectrolyte pumps 30 and 32 through the negative electrode compartment20 side and the positive electrode compartment 22 side of the redox flowbattery cell 18, respectively.

The redox flow battery system 10 may also include a first bipolar plate36 and a second bipolar plate 38, each positioned along a rear-facingside, e.g., opposite of a side facing the separator 24, of the negativeelectrode 26 and the positive electrode 28, respectively. The firstbipolar plate 36 may be in contact with the negative electrode 26 andthe second bipolar plate 38 may be in contact with the positiveelectrode 28. In other examples, however, the bipolar plates 36 and 38may be arranged proximate but spaced away from the electrodes 26 and 28and housed within the respective electrode compartments 20 and 22. Ineither case, the bipolar plates 36 and 38 may be electrically coupled tothe terminals 40 and 42, respectively, either via direct contacttherewith or through the negative and positive electrodes 26 and 28,respectively. The IFB electrolytes may be transported to reaction sitesat the negative and positive electrodes 26 and 28 by the first andsecond bipolar plates 36 and 38, resulting from conductive properties ofa material of the bipolar plates 36 and 38. Electrolyte flow may also beassisted by the negative and positive electrolyte pumps 30 and 32,facilitating forced convection through the redox flow battery cell 18.Reacted electrochemical species may also be directed away from thereaction sites by a combination of forced convection and a presence ofthe first and second bipolar plates 36 and 38.

As illustrated in FIG. 1 , the redox flow battery cell 18 may furtherinclude the negative battery terminal 40 and the positive batteryterminal 42. When a charge current is applied to the battery terminals40 and 42, the positive electrolyte may be oxidized (loses one or moreelectrons) at the positive electrode 28, and the negative electrolytemay be reduced (gains one or more electrons) at the negative electrode26. During battery discharge, reverse redox reactions may occur on theelectrodes 26 and 28. In other words, the positive electrolyte may bereduced (gains one or more electrons) at the positive electrode 28, andthe negative electrolyte may be oxidized (loses one or more electrons)at the negative electrode 26. An electrical potential difference acrossthe battery may be maintained by the electrochemical redox reactions inthe positive electrode compartment 22 and the negative electrodecompartment 20, and may induce an electric current through a currentcollector while the reactions are sustained. An amount of energy storedby a redox battery may be limited by an amount of electroactive materialavailable in electrolytes for discharge, depending on a total volume ofelectrolytes and a solubility of the electroactive materials.

In some examples, the redox flow battery cell 18 may be configured asone MEA 102 (or as the MEA described in detail below with reference toFIGS. 2B-5C) of an MEA stack (or the MEA stack described in detail belowwith reference to FIG. 2A). The MEA 102 may include an extrudedthermoplastic housing 104 including an integrated membrane sheet as theseparator 24. The integrated membrane sheet may be welded to an interiorsurface of the extruded thermoplastic housing 104 so as to separate thenegative and positive electrode compartments 20 and 22 respectivelyhousing the negative and positive electrodes 26 and 28. The extrudedthermoplastic housing 104 may further include integrated conductivesheets (e.g., the first and second bipolar plates 36 and 38 and/oradditional conductive sheets provided to lower resistance betweenbipolar plates 36, 38 on adjacent MEAs 102 of the MEA stack)electrically coupled to the battery terminals 40 and 42. The integratedconductive sheets may be welded to exterior surfaces of the extrudedthermoplastic housing 104 so as to form sides of the negative andpositive electrode compartments 20 and 22 opposite to the integratedmembrane sheet and thereby maintain electronic contact with the positiveand negative electrolytes respectively circulating through the negativeand positive electrode compartments 20 and 22.

The extruded thermoplastic housing 104 may be configured to expand orinflate during operation of the redox flow battery system 10 andcontract or deflate when the redox flow battery system 10 is not beingoperated. For example, the extruded thermoplastic housing 104 mayinflate responsive to applied internal pressure (e.g., internal pressurearising from pumping the positive and negative electrolytestherethrough) to increase an internal volume of the MEA 102 duringoperation of the redox flow battery system 10. The positive and negativeelectrolytes may be distributed through the MEA stack via common fluidmanifolds formed from fluidic couplings of mechanically interlockingelectrolyte distribution inserts configured in electrolyte ports of theMEAs 102 included in the MEA stack. Further, a flexibility of theextruded thermoplastic housing 104 may facilitate diagnosis, removal,and/or replacement of degraded or outmoded components within theextruded thermoplastic housing 104, as the extruded thermoplastichousing 104 may be relatively easily cut and opened to access thevarious components housed therein. In this way, the redox flow batterysystem 10 may include a plurality of MEAs 102 respectively operated asredox flow battery cells 18 and fluidically coupled to one another in aflexible, modular configuration (in other examples, however, only oneMEA 102 may be included in the redox flow battery system 10).

The redox flow battery system 10 may further include the integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedelectrolyte storage tank 110 may be divided by a bulkhead 98. Thebulkhead 98 may create multiple chambers within the multi-chamberedelectrolyte storage tank 110 so that both the positive and negativeelectrolytes may be included within a single tank. The negativeelectrolyte chamber 50 holds negative electrolyte including theelectroactive materials, and the positive electrolyte chamber 52 holdspositive electrolyte including the electroactive materials. The bulkhead98 may be positioned within the multi-chambered electrolyte storage tank110 to yield a desired volume ratio between the negative electrolytechamber 50 and the positive electrolyte chamber 52. In one example, thebulkhead 98 may be positioned to set a volume ratio of the negative andpositive electrolyte chambers 50 and 52 according to a stoichiometricratio between the negative and positive redox reactions. FIG. 1 furtherillustrates a fill height 112 of the multi-chambered electrolyte storagetank 110, which may indicate a liquid level in each tank compartment.FIG. 1 also shows a gas head space 90 located above the fill height 112of the negative electrolyte chamber 50, and a gas head space 92 locatedabove the fill height 112 of the positive electrolyte chamber 52. Thegas head space 92 may be utilized to store H₂ gas generated throughoperation of the redox flow battery (e.g., due to proton reduction andiron corrosion side reactions) and conveyed to the multi-chamberedelectrolyte storage tank 110 with returning electrolyte from the redoxflow battery cell 18. The H₂ gas may be separated spontaneously at agas-liquid interface (e.g., the fill height 112) within themulti-chambered electrolyte storage tank 110, thereby precluding havingadditional gas-liquid separators as part of the redox flow batterysystem 10. Once separated from the electrolyte, the H₂ gas may fill thegas head spaces 90 and 92. As such, the stored H₂ gas may aid in purgingother gases from the multi-chambered electrolyte storage tank 110,thereby acting as an inert gas blanket for reducing oxidation ofelectrolyte species, which may help to reduce redox flow batterycapacity losses. In this way, utilizing the integrated multi-chamberedelectrolyte storage tank 110 may forego having separate negative andpositive electrolyte storage tanks, hydrogen storage tanks, andgas-liquid separators common to conventional redox flow battery systems,thereby simplifying a system design, reducing a physical footprint ofthe redox flow battery system 10, and reducing system costs.

FIG. 1 also shows a spillover hole 96, which may create an opening inthe bulkhead 98 between the gas head spaces 90 and 92, and may provide ameans of equalizing gas pressure between the chambers 50 and 52. Thespillover hole 96 may be positioned at a threshold height above the fillheight 112. The spillover hole 96 may further enable a capability toself-balance the electrolytes in each of the negative and positiveelectrolyte chambers 50 and 52 in the event of a battery crossover. Inthe case of an all-iron redox flow battery system, the same electrolyte(Fe²⁺) is used in both negative and positive electrode compartments 20and 22, so spilling over of electrolyte between the negative andpositive electrolyte chambers 50 and 52 may reduce overall systemefficiency, but overall electrolyte composition, battery moduleperformance, and battery module capacity may be maintained. Flangefittings may be utilized for all piping connections for inlets andoutlets to and from the multi-chambered electrolyte storage tank 110 tomaintain a continuously pressurized state without leaks. Themulti-chambered electrolyte storage tank 110 may include at least oneoutlet from each of the negative and positive electrolyte chambers 50and 52, and at least one inlet to each of the negative and positiveelectrolyte chambers 50 and 52. Furthermore, one or more outletconnections may be provided from the gas head spaces 90 and 92 fordirecting H₂ gas to rebalancing reactors or cells 80 and 82.

Although not shown in FIG. 1 , the integrated multi-chamberedelectrolyte storage tank 110 may further include one or more heatersthermally coupled to each of the negative electrolyte chamber 50 and thepositive electrolyte chamber 52. In alternate examples, only one of thenegative and positive electrolyte chambers 50 and 52 may include one ormore heaters. In the case where only the positive electrolyte chamber 52includes one or more heaters, the negative electrolyte may be heated bytransferring heat generated at the redox flow battery cell 18 to thenegative electrolyte. In this way, the redox flow battery cell 18 mayheat and facilitate temperature regulation of the negative electrolyte.The one or more heaters may be actuated by a controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52 independently or together. For example, inresponse to an electrolyte temperature decreasing below a thresholdtemperature, the controller 88 may increase a power supplied to one ormore heaters so that a heat flux to the electrolyte may be increased.The electrolyte temperature may be indicated by one or more temperaturesensors mounted at the multi-chambered electrolyte storage tank 110,such as sensors 60 and 62. As examples, the one or more heaters mayinclude coil type heaters or other immersion heaters immersed in theelectrolyte fluid, or surface mantle type heaters that transfer heatconductively through the walls of the negative and positive electrolytechambers 50 and 52 to heat the fluid therein. Other known types of tankheaters may be employed without departing from the scope of the presentdisclosure. Furthermore, the controller 88 may deactivate the one ormore heaters in the negative and positive electrolyte chambers 50 and 52in response to a liquid level decreasing below a solids fill thresholdlevel. Said in another way, in some examples, the controller 88 mayactivate the one or more heaters in the negative and positiveelectrolyte chambers 50 and 52 only in response to a liquid levelincreasing above the solids fill threshold level. In this way,activating the one or more heaters without sufficient liquid in thenegative and/or positive electrolyte chambers 50, 52 may be averted,thereby reducing a risk of overheating or burning out the heater(s).

Further still, one or more inlet connections may be provided to each ofthe negative and positive electrolyte chambers 50 and 52 from a fieldhydration system (not shown). In this way, the field hydration systemmay facilitate commissioning of the redox flow battery system 10,including installing, filling, and hydrating the redox flow batterysystem 10, at an end-use location. Furthermore, prior to commissioningthe redox flow battery system 10 at the end-use location, the redox flowbattery system 10 may be dry-assembled at a battery manufacturingfacility different from the end-use location without filling andhydrating the redox flow battery system 10, before delivering the redoxflow battery system 10 to the end-use location. In one example, theend-use location may correspond to a location where the redox flowbattery system 10 is to be installed and utilized for on-site energystorage. Said another way, the redox flow battery system 10 may bedesigned such that, once installed and hydrated at the end-use location,a position of the redox flow battery system 10 may become fixed, and theredox flow battery system 10 may no longer be deemed a portable, drysystem. Thus, from a perspective of an end-user, the dry, portable redoxflow battery system 10 may be delivered on-site, after which the redoxflow battery system 10 may be installed, hydrated, and commissioned.Prior to hydration, the redox flow battery system 10 may be referred toas a dry, portable system, the redox flow battery system 10 being freeof or without water and wet electrolyte. Once hydrated, the redox flowbattery system 10 may be referred to as a wet, non-portable system, theredox flow battery system 10 including wet electrolyte.

Further illustrated in FIG. 1 , electrolyte solutions primarily storedin the multi-chambered electrolyte storage tank 110 may be pumped viathe negative and positive electrolyte pumps 30 and 32 throughout theredox flow battery system 10. Electrolyte stored in the negativeelectrolyte chamber 50 may be pumped via the negative electrolyte pump30 through the negative electrode compartment 20 side of the redox flowbattery cell 18, and electrolyte stored in the positive electrolytechamber 52 may be pumped via the positive electrolyte pump 32 throughthe positive electrode compartment 22 side of the redox flow batterycell 18.

The electrolyte rebalancing reactors 80 and 82 may be connected in lineor in parallel with the recirculating flow paths of the electrolyte atthe negative and positive sides of the redox flow battery cell 18,respectively, in the redox flow battery system 10. One or morerebalancing reactors may be connected in-line with the recirculatingflow paths of the electrolyte at the negative and positive sides of thebattery, and other rebalancing reactors may be connected in parallel,for redundancy (e.g., a rebalancing reactor may be serviced withoutdisrupting battery and rebalancing operations) and for increasedrebalancing capacity. In one example, the electrolyte rebalancingreactors 80 and 82 may be placed in a return flow path from the negativeand positive electrode compartments 20 and 22 to the negative andpositive electrolyte chambers 50 and 52, respectively. In some examples,the electrolyte rebalancing reactors 80 and 82 may be respectivelyconfigured as rebalancing cell assemblies 80 and 82 stacked with oneanother and/or with the MEA stack and fluidically coupled (e.g., via thecommon fluid manifolds) to the MEAs 102 being operated as the redox flowbattery cells 18 therein. In certain examples, the electrolyterebalancing reactors 80 and 82 may mechanically interlock with adjacentMEAs 102 via the interlocking electrolyte distribution inserts.

The electrolyte rebalancing reactors 80 and 82 may serve to rebalanceelectrolyte charge imbalances in the redox flow battery system 10occurring due to side reactions, ion crossover, and the like, asdescribed herein. In one example, electrolyte rebalancing reactors 80and 82 may include trickle bed reactors, where the H₂ gas andelectrolyte may be contacted at catalyst surfaces in a packed bed forcarrying out the electrolyte rebalancing reaction. In other examples,the rebalancing reactors 80 and 82 may include flow-through typereactors that are capable of contacting the H₂ gas and the electrolyteliquid and carrying out the electrolyte rebalancing reactions absent apacked catalyst bed.

During operation of the redox flow battery system 10, sensors and probesmay monitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, SOC, and the like. For example, asillustrated in FIG. 1 , sensors 62 and 60 maybe be positioned to monitorpositive electrolyte and negative electrolyte conditions at the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50,respectively. In another example, sensors 62 and 60 may each include oneor more electrolyte level sensors to indicate a level of electrolyte inthe positive electrolyte chamber 52 and the negative electrolyte chamber50, respectively. As another example, sensors 72 and 70, alsoillustrated in FIG. 1 , may monitor positive electrolyte and negativeelectrolyte conditions at the positive electrode compartment 22 and thenegative electrode compartment 20, respectively. The sensors 72 and 70may be pH probes, optical probes, pressure sensors, voltage sensors,etc. It will be appreciated that sensors may be positioned at otherlocations throughout the redox flow battery system 10 to monitorelectrolyte chemical properties and other properties.

For example, a sensor may be positioned in an external acid tank (notshown) to monitor acid volume or pH of the external acid tank, whereinacid from the external acid tank may be supplied via an external pump(not shown) to the redox flow battery system 10 in order to reduceprecipitate formation in the electrolytes. Additional external tanks andsensors may be installed for supplying other additives to the redox flowbattery system 10. For example, various sensors including, temperature,conductivity, and level sensors of a field hydration system may transmitsignals to the controller 88. Furthermore, the controller 88 may sendsignals to actuators such as valves and pumps of the field hydrationsystem during hydration of the redox flow battery system 10. Sensorinformation may be transmitted to the controller 88 which may in turnactuate the pumps 30 and 32 to control electrolyte flow through theredox flow battery cell 18, or to perform other control functions, as anexample. In this manner, the controller 88 may be responsive to one or acombination of sensors and probes.

The redox flow battery system 10 may further include a source of H₂ gas.In one example, the source of H₂ gas may include a separate dedicatedhydrogen gas storage tank. In the example of FIG. 1 , H₂ gas may bestored in and supplied from the integrated multi-chambered electrolytestorage tank 110. The integrated multi-chambered electrolyte storagetank 110 may supply additional H₂ gas to the positive electrolytechamber 52 and the negative electrolyte chamber 50. The integratedmulti-chambered electrolyte storage tank 110 may alternately supplyadditional H₂ gas to an inlet of the electrolyte rebalancing reactors 80and 82. As an example, a mass flow meter or other flow controllingdevice (which may be controlled by the controller 88) may regulate flowof the H₂ gas from the integrated multi-chambered electrolyte storagetank 110. The integrated multi-chambered electrolyte storage tank 110may supplement the H₂ gas generated in the redox flow battery system 10.For example, when gas leaks are detected in the redox flow batterysystem 10 or when a reduction reaction rate is too low at low hydrogenpartial pressure, the H₂ gas may be supplied from the integratedmulti-chambered electrolyte storage tank 110 in order to rebalance theSOC of the electroactive materials in the positive electrolyte and thenegative electrolyte. As an example, the controller 88 may supply the H₂gas from the integrated multi-chambered electrolyte storage tank 110 inresponse to a measured change in pH or in response to a measured changein SOC of an electrolyte or an electroactive material.

For example, an increase in pH of the negative electrolyte chamber 50,or the negative electrode compartment 20, may indicate that H₂ gas isleaking from the redox flow battery system 10 and/or that the reactionrate is too slow with the available hydrogen partial pressure, and thecontroller 88, in response to the pH increase, may increase a supply ofH₂ gas from the integrated multi-chambered electrolyte storage tank 110to the redox flow battery system 10. As a further example, thecontroller 88 may supply H₂ gas from the integrated multi-chamberedelectrolyte storage tank 110 in response to a pH change, wherein the pHincreases beyond a first threshold pH or decreases beyond a secondthreshold pH. In the case of an IFB, the controller 88 may supplyadditional H₂ gas to increase a rate of reduction of Fe³⁺ ions and arate of production of protons, thereby reducing the pH of the positiveelectrolyte. Furthermore, the pH of the negative electrolyte may belowered by hydrogen reduction of Fe³⁺ ions crossing over from thepositive electrolyte to the negative electrolyte or by protons,generated at the positive side, crossing over to the negativeelectrolyte due to a proton concentration gradient and electrophoreticforces. In this manner, the pH of the negative electrolyte may bemaintained within a stable region, while reducing the risk ofprecipitation of Fe³⁺ ions (crossing over from the positive electrodecompartment 22) as Fe(OH)₃.

Other control schemes for controlling a supply rate of H₂ gas from theintegrated multi-chambered electrolyte storage tank 110 responsive to achange in an electrolyte pH or to a change in an electrolyte SOC,detected by other sensors such as an oxygen-reduction potential (ORP)meter or an optical sensor, may be implemented. Further still, thechange in pH or SOC triggering action of the controller 88 may be basedon a rate of change or a change measured over a time period. The timeperiod for the rate of change may be predetermined or adjusted based ontime constants for the redox flow battery system 10. For example, thetime period may be reduced if a recirculation rate is high, and localchanges in concentration (e.g., due to side reactions or gas leaks) mayquickly be measured since the time constants may be small.

The controller 88 may further execute control schemes based on anoperating mode of the redox flow battery system 10. For example, thecontroller 88 may control charging and discharging of the redox flowbattery cell 18 so as to cause iron preformation at the negativeelectrode 26 during system conditioning (where system conditioning mayinclude an operating mode employed to optimize electrochemicalperformance of the redox flow battery system 10 outside of batterycycling). That is, during system conditioning, the controller 88 mayadjust one or more operating conditions of the redox flow battery system10 to plate iron metal on the negative electrode 26 to improve a batterycharge capacity during subsequent battery cycling (thus, the iron metalmay be preformed for battery cycling). The controller 88 may furtherexecute electrolyte rebalancing as discussed above to rid the redox flowbattery system 10 of excess hydrogen gas and reduce Fe³⁺ ionconcentration. In this way, preforming iron at the negative electrode 26and running electrolyte rebalancing during the system conditioning mayincrease an overall capacity of the redox flow battery cell 18 duringbattery cycling by mitigating iron plating loss. As used herein, batterycycling (also referred to as “charge cycling”) may include alternatingbetween a charging mode and a discharging mode of the redox flow batterysystem 10.

It will be appreciated that all components apart from the sensors 60 and62 and the integrated multi-chambered electrolyte storage tank 110 (andcomponents included therein) may be considered as being included in apower module 120. As such, the redox flow battery system 10 may bedescribed as including the power module 120 fluidly coupled to theintegrated multi-chambered electrolyte storage tank 110 and communicablycoupled to the sensors 60 and 62. In some examples, each of the powermodule 120 and the multi-chambered electrolyte storage tank 110 may beincluded in a single housing (not shown), such that the redox flowbattery system 10 may be contained as a single unit in a singlelocation. It will further be appreciated the positive electrolyte, thenegative electrolyte, the sensors 60 and 62, the electrolyte rebalancingreactors 80 and 82, and the integrated multi-chambered electrolytestorage tank 110 (and components included therein) may be considered asbeing included in an electrolyte subsystem 130. As such, the electrolytesubsystem 130 may supply one or more electrolytes to the redox flowbattery cell 18 (and components included therein).

Referring now to FIG. 2A, an exploded view 200 depicting an MEA stack202 for a redox flow battery system, such as the redox flow batterysystem 10 of FIG. 1 , is shown. In an exemplary embodiment, the MEAstack 202 may include a sequential stack of MEAs 204 which may beoperated as redox flow battery cells. Accordingly, each of the MEAs 204may be configured as the MEA 102 capable of being operated as the redoxflow battery cell 18 of FIG. 1 . A set of reference axes 201 is furtherprovided for describing relative positioning of the components shown andfor comparison between the views of FIGS. 2A-5C, the axes 201 indicatingan x-axis, a y-axis, and a z-axis. For example, the terms “upper” and“lower” when qualifying relative component positioning herein may referto upper and lower z-axis positions, respectively.

In some examples, the MEAs 204 may be formed via roll-to-rollprocessing. In such examples, due to decreased tolerance stackingafforded by the roll-to-roll processing, a number of MEAs 204 in the MEAstack 202 may be increased relative to an electrode assembly stackmanufactured via a molding-based electrode assembly process. Forexample, the MEA stack 202 is shown in the exploded view 200 as asequential stack of 50 MEAs 204. However, the number of MEAs 204included in the MEA stack 202 is not particularly limited and greater orfewer MEAs 204 may be included in the MEA stack 202 according toperformance demands of a particular application. However, in someexamples, relatively large numbers of MEAs 204 may includecorrespondingly greater shunt protection (e.g., by increasing aneffective length of anti-shunting flow paths therein, such as theanti-shunting flow paths discussed in detail below with reference toFIGS. 5A-5C) and larger packaging. Accordingly, in such examples, anupper limit to the number of MEAs 204 may depend on performance issuesascribed to increased pressure drops from excessively long anti-shuntingflow paths (the anti-shunting flow paths may in turn be limited bypractical packaging constraints).

As shown, each MEA 204 may include an external enclosure or housing 210at least partially enclosing various components of the MEA 204. Thehousing 210 may be composed of a material having a low electricalconductivity, such as a plastic or other polymer, so as to reduceundesirable shorting events. For example, the housing 210 may becomposed of extruded thermoplastic sheets or frames welded or otherwiseadhered to one another (as used herein, “adhered” may refer to bondingbetween two components). In some examples, the extruded thermoplasticsheets or frames may be composed of a material selected from highdensity polyethylene (HDPE), polypropylene, and fiber-reinforcedformulations of any of the preceding materials (e.g., to conferadditional structural strength). In other examples, each of a thicknessand a degree of fiber reinforcement of the extruded thermoplastic sheetsor frames may be increased to correspondingly increase a stiffness orstructural strength of the extruded thermoplastic sheets or frames(e.g., for outer load-bearing sheets or frames, such as an upperconductive sheet 260 and/or the lower conductive sheet 290 described indetail below with reference to FIGS. 2B and 3 ). Similarly, in someinstances, each of the thickness and the degree of fiber reinforcementof the extruded thermoplastic sheets or frames may be decreased tocorrespondingly increase a flexibility of the extruded thermoplasticsheets or frames (e.g., for inner sheets, such as the membrane sheet 270described in detail below with reference to FIGS. 2B and 3 ).

The extruded thermoplastic sheets or frames may be reversiblyexpandable, such that compartments, chambers, or other cavities formingthe internal volume may be expanded or inflated from a baseconfiguration to an inflated configuration when the MEAs 204 are beingoperated as redox flow battery cells. The MEAs 204 may also becontracted or deflated from the inflated configuration back to the baseconfiguration when the MEAs 204 are not being operated as redox flowbattery cells. In some examples, the extruded thermoplastic sheets orframes forming the housing 210 may be adhered to one another and toother components such that an internal volume of the MEA 204 (such asthe internal volume 310 described in detail below with reference to FIG.3 ) is hermetically sealed during expansion or inflation of the housing210 excepting at a plurality of electrolyte ports (described in detailbelow).

For instance, conductive sheets of each MEA 204 may be welded orotherwise adhered to opposite sides 212 a and 212 b of the housing 210,the conductive sheets sealing upper and lower openings (such as thecentral openings 265 and 295 described in detail below with reference toFIG. 2B) disposed within the housing 210 around perimeters thereof. Inone example, the conductive sheets may hermetically seal the upper andlower openings disposed within the housing 210 around the perimetersthereof so as to prevent electrolyte leakage thereat. The conductivesheets may be composed of a conductive material which may affordadditional structural stability to the housing 210, such as carbonfiber, and may be electrically coupled to terminals of the redox flowbattery system. In the exploded view 200, only upper conductive sheets260 of some MEAs 204 are visible, positioned on the (upper) sides 212 aof the housings 210 of respective MEAs 204; however, lower conductivesheets may be included on the (lower) sides 212 b of the housings 210 ofthe respective MEAs 204 (such as the lower conductive sheet 290described in detail below with reference to FIGS. 2B and 3 ). Further,all of the MEAs 204 may include, on respective sides 212 a thereof,respective upper conductive sheets 260 (whether or not visible in theexploded view 200).

The conductive sheets of each MEA 204 may respectively form sides of thecompartments, chambers, or other cavities forming the internal volume ofthe MEA 204. For example, the upper conductive sheet 260 of each MEA 204may form a side of a corresponding positive electrode compartment (suchas the positive electrode compartment 312 described in detail below withreference to FIG. 3 ) and the lower conductive sheet (not shown at FIG.2A; see FIGS. 2B and 3 ) of each MEA 204 may form a side of acorresponding negative electrode compartment (such as the negativeelectrode compartment 314 described in detail below with reference toFIG. 3 ). Further, during operation of the MEAs 204 as redox flowbattery cells, electric current may be applied across the conductivesheets via terminals of the redox flow battery system electricallycoupled thereto (such as the terminals 40 and 42 described in detailabove with respect to FIG. 1 ). In this way, the conductive sheets maybe in both fluidic and electrical communication with the internal volumeof the MEA 204 (e.g., with liquids or other fluids in the internalvolume, such as positive and negative electrolytes, in contact with theconductive sheets).

In some examples, respective conductive sheets of pairs of adjacent MEAs204 may be in face-sharing contact with one another. As an example, alower conductive sheet of an MEA 204 at an upper end 214 a of the MEAstack 202 (e.g., at a highest value along the z-axis) may be inface-sharing contact with an upper conductive sheet 260 of an adjacentMEA 204 (e.g., a nearest MEA 204 in a negative direction along thez-axis). As another example, an upper conductive sheet 260 of an MEA 204at a lower end 214 b of the MEA stack 202 (e.g., at a lowest value alongthe z-axis) may be in face-sharing contact with a lower conductive sheetof an adjacent MEA 204 (e.g., a nearest MEA 204 in a positive directionalong the z-axis). As yet another example, for each given MEA 204 inbetween the MEA 204 at the upper end 214 a of the MEA stack 202 and theMEA 204 at the lower end 214 b of the MEA stack 202, a lower conductivesheet of the given MEA 204 may be in face-sharing contact with an upperconductive sheet 260 of an adjacent MEA 204 (e.g., a nearest MEA 204 ina positive direction along the z-axis) and an upper conductive sheet 260of the given MEA 204 may be in face-sharing contact with a lowerconductive sheet of another adjacent MEA 204 (e.g., a nearest MEA 204 ina negative direction along the z-axis).

In some examples, respective conductive sheets of pairs of adjacent MEAs204 may only be in face-sharing contact with one another when the MEAs204 are being operated as redox flow battery cells (e.g., when thehousing 210 of each MEA 204 may be expanded or inflated, such that therespective conductive sheets of pairs of adjacent MEAs 204 are pressedagainst one another). Correspondingly, in such examples, the respectiveconductive sheets of pairs of adjacent MEAs 204 may not be inface-sharing contact with one another when the MEAs 204 are not beingoperated as redox flow battery cells (e.g., when the housing 210 of eachMEA 204 may be contracted or deflated, such that the respectiveconductive sheets of pairs of adjacent MEAs 204 are not pressed againstone another and a space exists therebetween). In additional oralternative examples, an electrical contact resistance between adjacentpairs of MEAs 204 while the MEAs 204 are being operated as redox flowbattery cells may be lower than an electrical contact resistance betweenadjacent pairs of MEAs 204 while the MEAs 204 are not being operated asredox flow battery cells (e.g., because the respective conductive sheetsof pairs of adjacent MEAs 204 may be pressed against one another duringoperation of the MEAs 204 as redox flow battery cells). Accordingly,conductive sheets of adjacent MEAs 204, being in physical contact withone another and having a relatively low electrical contact resistancetherebetween, may conduct electricity in series.

Each given MEA 204 may include one or more electrolyte ports formed byaligning openings in each of the extruded thermoplastic sheets or framesof the given MEA 204. For example, in the exploded view 200, each of theMEAs 204 may include each of first positive electrolyte port 206 a,second positive electrolyte port 206 b, first negative electrolyte port208 a, and second negative electrolyte port 208 b fluidically coupled tothe internal volume. In one example, each of the first and secondpositive electrolyte ports 206 a, 206 b may be fluidically coupled tothe positive electrode compartment (e.g., via respective positiveelectrolyte passages formed between two of the extruded thermoplasticsheets or frames, as discussed in detail below with reference to FIG.5C) and each of the first and second negative electrolyte ports 208 a,208 b may be fluidically coupled to the negative electrode compartment(e.g., via respective negative electrolyte passages formed between twoof the extruded thermoplastic sheets or frames, as discussed in detailbelow with reference to FIG. 5C).

In some examples, one of the first and second positive electrolyte ports206 a, 206 b may serve as a positive electrolyte inlet, while the otherone of the first and second positive electrolyte ports 206 a, 206 b mayserve as a positive electrolyte outlet, where the positive electrolytemay enter into a given MEA 204 via the positive electrolyte inlet, flowthrough the positive electrode compartment of the given MEA 204, andexit the given MEA 204 via the positive electrolyte outlet. Similarly,in some examples, one of the first and second negative electrolyte ports208 a, 208 b may serve as a negative electrolyte inlet, while the otherone of the first and second negative electrolyte ports 208 a, 208 b mayserve as a negative electrolyte outlet, where the negative electrolytemay enter into a given MEA 204 via the negative electrolyte inlet, flowthrough the negative electrode compartment of the given MEA 204, andexit the given MEA 204 via the negative electrolyte outlet.

The one or more electrolyte ports of each of the MEAs 204 may be alignedto form one or more electrolyte manifolds, respectively, fluidicallycoupling each of the MEAs 204 (e.g., the internal volume of each of theMEAs 204) to one another. For example, in the exploded view 200, a firstpositive electrolyte manifold 216 a may be formed by aligning each ofthe first positive electrolyte ports 206 a of the MEAs 204 along thez-axis, a second positive electrolyte manifold 216 b may be formed byaligning each of the second positive electrolyte ports 206 b of the MEAs204 along the z-axis, a first negative electrolyte manifold 218 a may beformed by aligning each of the first negative electrolyte ports 208 a ofthe MEAs 204 along the z-axis, and a second negative electrolytemanifold 218 b may be formed by aligning each of the second negativeelectrolyte ports 208 b of the MEAs 204 along the z-axis. In someexamples, one of the first and second positive electrolyte manifolds 216a, 216 b may serve as a positive electrolyte inlet manifold, while theother one of the first and second positive electrolyte manifolds 216 a,216 b may serve as a positive electrolyte outlet manifold, where thepositive electrolyte may enter into the MEA stack 202 via the positiveelectrolyte inlet manifold, flow through the positive electrodecompartments of each of the MEAs 204, and exit the MEA stack 202 via thepositive electrolyte outlet manifold. Similarly, in some examples, oneof the first and second negative electrolyte manifolds 218 a, 218 b mayserve as a negative electrolyte inlet manifold, while the other one ofthe first and second negative electrolyte manifolds 218 a, 218 b mayserve as a negative electrolyte outlet manifold, where the negativeelectrolyte may enter into the MEA stack 202 via the negativeelectrolyte inlet manifold, flow through the negative electrodecompartments of each of the MEAs 204, and exit the MEA stack 202 via thenegative electrolyte outlet manifold.

In some examples, and as discussed in detail below with reference toFIGS. 2B and 4A-4C, the first and second positive electrolyte manifolds216 a, 216 b and the first and second negative electrolyte manifolds 218a, 218 b may be formed by mechanically interlocking electrolytedistribution inserts respectively included in the one or moreelectrolyte ports of each MEA 204. In an exemplary embodiment,electrolyte distribution inserts of adjacent pairs of MEAs 204 maymechanically interlock with one another, thereby hermetically sealingthe electrolyte manifolds formed therefrom (e.g., such that the positiveor negative electrolyte may not leak between pairs of mechanicallyinterlocking electrolyte distribution inserts). Accordingly, byhermetically sealing the electrolyte manifolds in this way, spaces orinterfaces between the MEAs 204 may be dry and substantially free of thepositive and negative electrolytes.

By configuring the MEAs 204 to mechanically interlock with one another(e.g., via pairs of the electrolyte distribution inserts), the MEA stack202 may be formed in a modular fashion, such that MEAs 204 may be added,removed, replaced, or substituted according to a given application. Asan example, if an MEA 204 is determined to be degraded, the MEA 204 maybe mechanically decoupled from the MEA stack 202 and replaced with a new(non-degraded) MEA 204. As another example, assemblies having differingconfigurations from the MEA 204 may be stacked within or on the MEAstack 202.

For instance, the redox flow battery system may include one or morerebalancing cell assemblies (e.g., where at least some of the one ormore rebalancing cells may be independently configured as either of therebalancing reactors 80, 82 of FIG. 1 ) arranged in the sequential stackwith the MEAs 204 (e.g., stacked on the MEA stack 202), where at leastone of the one or more rebalancing cell assemblies may mechanicallyinterlock with at least one of the MEAs 204 via pairs of the electrolytedistribution inserts (thereby fluidically coupling the one or morerebalancing cell assemblies to the MEA stack 202 via at least one of theelectrolyte manifolds). For example, on one side of the givenrebalancing cell assembly, electrolyte distribution insert(s) of a givenrebalancing cell assembly may mechanically interlock with respectiveelectrolyte distribution insert(s) of an adjacent MEA 204 (or anadjacent rebalancing cell assembly). On an opposite side of the givenrebalancing cell assembly, the electrolyte distribution insert(s) of thegiven rebalancing cell assembly may mechanically interlock withrespective electrolyte distribution insert(s) of another adjacentrebalancing cell assembly (or the electrolyte distribution insert(s) onthe opposite side of the given rebalancing cell assembly may directlyfluidically couple, e.g., with no intervening rebalancing cellassemblies, with an electrolyte subsystem of the redox flow batterysystem).

Accordingly, in some examples, the one or more rebalancing cellassemblies may be stacked in a rebalancing cell assembly substackseparate from the MEA stack 202, such that only one of the one or morerebalancing cell assemblies (e.g., at a lower or upper end of therebalancing cell assembly substack) may mechanically interlock with onlyone of the MEAs 204 (e.g., at an upper or lower end of the MEA stack202). Positioning the one or more rebalancing cell assemblies in closeproximity with the MEA stack 202 in this way may be advantageous in thatside reactions in the electrolytes which may be destabilized atoperating pH levels may be rapidly re-stabilized or maintained byavoiding lengthy electrolyte flow paths between the MEA stack 202 andthe one or more rebalancing cell assemblies. The MEAs 204 of the MEAstack 202 may drive relatively large electric currents thereacross atlevels impractical for the one or more rebalancing cell assemblies.Accordingly, in an exemplary embodiment, the one or more rebalancingcells may be electrically decoupled from the MEA stack 202 whileremaining fluidically coupled thereto (e.g., at the upper or lower endof the MEA stack 202).

In certain examples, the one or more rebalancing cell assemblies may notbe directly mechanically coupled to the MEA stack 202 (e.g., pipingand/or other components of the redox flow battery system may bepositioned between the one or more rebalancing cell assemblies and theMEA stack 202), such that the one or more rebalancing cell assembliesmay be packaged separately from the MEA stack 202. In such examples,additional hardware (e.g., piping, pressure plates to contain excessfluid pressures, etc.) may be implemented and greater floor space may beutilized as compared to examples wherein the one or more rebalancingcell assemblies are mechanically interlocked with the MEA stack 202 viapairs of the electrolyte distribution inserts.

Referring now to FIG. 2B, an exploded view 250 depicting the MEA 204 isshown. In an exemplary embodiment, the MEA 204 may be one of the MEAs204 of the MEA stack 202 of FIG. 2A. Accordingly, in some examples, theMEA 204 may be operated as a redox flow battery cell in a redox flowbattery system.

The MEA 204 may include a housing (e.g., the housing 210 of FIG. 2A)formed from one or more extruded thermoplastic sheets or frames weldedor otherwise adhered to one another. In some examples, the one or moreextruded thermoplastic sheets or frames may include an upperthermoplastic frame 262, a middle thermoplastic frame 272, and a lowerthermoplastic frame 292. In such examples, the upper thermoplastic frame262 may be welded or otherwise adhered to the middle thermoplastic frame272 to form the positive electrode compartment (as described in detailbelow with reference to FIG. 3 ). Similarly, in such examples, the lowerthermoplastic frame 292 may be welded or otherwise adhered to the middlethermoplastic frame 272 to form the negative electrode compartment (asdescribed in detail below with reference to FIG. 3 ).

Each of the one or more extruded thermoplastic sheets or frames mayinclude one or more openings disposed therein and extending therethroughalong the z-axis. For example, and as shown in the exploded view 250,the upper, middle, and lower thermoplastic frames 262, 272, and 292 mayrespectively include central openings 265, 275, and 295. The centralopenings 265, 275, and 295 are shown as having a rectangular shape(e.g., in a plane perpendicular to the z-axis). However, it will beappreciated that the shape of each of the central openings 265, 275, and295 may be independently configured in other polygonal shapes or incircular, elliptical, or irregular shapes as desired for a givenapplication. Further, though the central openings 265, 275, and 295 areshown in the exploded view 250 as having substantially equivalent sizes,in other examples, the central openings 265, 275, and 295 may vary insize.

In some examples, each of the central openings 265, 275, and 295 may besealed around respective perimeters thereof by corresponding sheets. Asan example, the upper conductive sheet 260 may be welded or otherwiseadhered to the upper thermoplastic frame 262, such that the centralopening 265 may be sealed (e.g., hermetically sealed) around theperimeter thereof. As another example, a lower conductive sheet 290 maybe welded or otherwise adhered to the lower thermoplastic frame 292,such that the central opening 295 may be sealed (e.g., hermeticallysealed) around the perimeter thereof. In some examples, each of theupper and lower conductive sheets 260, 290 may correspond to the samestructural configuration and composition (though relative positioning ofthe upper and lower conductive sheets 260, 290 may differ from oneanother). Accordingly, in such examples, each of the upper and lowerconductive sheets 260, 290 may include each of the structural andcompositional features described above with reference to FIG. 2A. Forexample, each of the upper and lower conductive sheets 260, 290 may becomposed of carbon fiber.

As yet another example, a membrane sheet 270 may be welded or otherwiseadhered to the middle thermoplastic frame 272, such that the centralopening 275 may be sealed (e.g., hermetically sealed) around theperimeter thereof. In doing so, the membrane sheet 270 may bisect theinternal volume of the MEA 204 so as to form the positive and negativeelectrode compartments. Accordingly, the membrane sheet 270 may bepositioned between the positive and negative compartments, forming aside of each of the positive and negative compartments. The membranesheet 270 may be composed of an electrically insulating ionic conductingbarrier (e.g., an ion-exchange membrane barrier, a microporous membrane,flexible porous ceramic separator, or the like, optionally coated withNation™ to mitigate fluid crossover) which prevents bulk mixing of thepositive and negative electrolytes while allowing conductance ofspecific ions therethrough. The electrically insulating ionic conductingbarrier may further be selected to withstand bonding forces andtemperatures endured during adherence of the membrane sheet 270 to themiddle thermoplastic frame 272.

A smallest dimension of each of the upper conductive sheet 260, themembrane sheet 270, and the lower conductive sheet 290 may beperpendicular to the z-axis, and an orthographic projection of each ofthe upper conductive sheet 260, the membrane sheet 270, and the lowerconductive sheet 290 onto a plane perpendicular to the z-axis may have ashape similar to, and slightly larger than, the shapes of the centralopenings 265, 275, and 295, respectively. For example, the shape of theorthographic projection of each of the upper conductive sheet 260, themembrane sheet 270, and the lower conductive sheet 290 may berectangular and slightly larger than the rectangular shapes of thecentral openings 265, 275, and 295, respectively. However, it will beappreciated that the shape of the orthographic projection of each of theupper conductive sheet 260, the membrane sheet 270, and the lowerconductive sheet 290 may be independently configured in other polygonalshapes or in circular, elliptical, or irregular shapes as desired for agiven application (e.g., to conform to a non-rectangular central opening265, 275, or 295, respectively). Further, though the upper conductivesheet 260, the membrane sheet 270, and the lower conductive sheet 290are shown in the exploded view 250 as having substantially equivalentsizes, in other examples, the upper conductive sheet 260, the membranesheet 270, and the lower conductive sheet 290 may vary in size.

As further shown in the exploded view 250, each of the upper, middle,and lower thermoplastic frames 262, 272, and 292 may further include oneor more port openings disposed therein and extending therethrough alongthe z-axis. In one example, the upper thermoplastic frame 262 mayinclude a first positive electrolyte port opening 266 a, a secondpositive electrolyte port opening 266 b, a first negative electrolyteport opening 268 a, and a second negative electrolyte port opening 268b. The middle thermoplastic frame 272 may include a first positiveelectrolyte port opening 276 a, a second positive electrolyte portopening 276 b, a first negative electrolyte port opening 278 a, and asecond negative electrolyte port opening 278 b. The lower thermoplasticframe 292 may include a first positive electrolyte port opening 296 a, asecond positive electrolyte port opening 296 b, a first negativeelectrolyte port opening 298 a, and a second negative electrolyte portopening 298 b.

The upper, middle, and lower thermoplastic frames 262, 272, and 292 maybe aligned along the z-axis such that the one or more port openings ineach of the upper, middle, and lower thermoplastic frames 262, 272, and292 may form one or more electrolyte ports, respectively. In oneexample, the first positive electrolyte port openings 266 a, 276 a, and296 a may align to form a first positive electrolyte port (e.g., thefirst positive electrolyte port 206 a of FIG. 2A), the second positiveelectrolyte port openings 266 b, 276 b, and 296 b may align to form asecond positive electrolyte port (e.g., the second positive electrolyteport 206 b of FIG. 2A), the first negative electrolyte port openings 268a, 278 a, and 298 a may align to form a first negative electrolyte port(e.g., the first negative electrolyte port 208 a of FIG. 2A), and thesecond negative electrolyte port openings 268 b, 278 b, and 298 b mayalign to form a second negative electrolyte port (e.g., the secondnegative electrolyte port 208 b of FIG. 2A).

The port openings are shown as having a circular shape (e.g., in a planeperpendicular to the z-axis). However, it will be appreciated that theshape of each of the port openings may be independently configured inelliptical, polygonal, or irregular shapes as desired for a givenapplication. Further, though the port openings are shown in the explodedview 250 as having substantially equivalent sizes, in other examples,the port openings may vary in size. In some examples, and as shown inthe exploded view 250, each of the port openings (e.g., 266 a, 266 b,268 a, 268 b, 276 a, 276 b, 278 a, 278 b, 296 a, 296 b, 298 a, 298 b)may be smaller in size than each of the central openings 265, 275, and295.

As further shown in the exploded view 250, electrolyte distributioninserts 282 may be respectively positioned between port openings ofadjacent extruded thermoplastic sheets or frames, the electrolytedistribution inserts 282 being held in place by adherence of theadjacent extruded thermoplastic sheets or frames to one another. In oneexample, at least a portion of the electrolyte distribution inserts 282may be positioned between the upper and middle thermoplastic frames 262,272. For instance, one of the electrolyte distribution inserts 282 maybe positioned between the first negative electrolyte port openings 268a, 278 a and another one of the electrolyte distribution inserts 282 maybe positioned between the second negative electrolyte port openings 268b, 278 b. Similarly, in an additional or alternative example, at least aportion of the electrolyte distribution inserts 282 may be positionedbetween the middle and lower thermoplastic frames 272, 292. Forinstance, one of the electrolyte distribution inserts 282 may bepositioned between the first positive electrolyte port openings 276 a,296 a and another one of the electrolyte distribution inserts 282 may bepositioned between the second positive electrolyte port openings 276 b,296 b.

Each given electrolyte distribution insert 282 may be disposedcircumferentially around the two port openings between which the givenelectrolyte distribution insert 282 is positioned. For example, when theport openings have circular shapes, the electrolyte distribution insertsmay be correspondingly annular. In this way, each of the electrolytedistribution inserts 282 may circumscribe a corresponding one of theelectrolyte ports.

As described in greater detail below with reference to FIGS. 4A-4C, aplurality of electrolyte channels may be disposed around a circumferenceof each of the electrolyte distribution inserts 282, fluidicallycoupling the electrolyte ports to electrolyte passages of the MEA 204.Upon the positive or negative electrolyte flowing into the MEA 204 via arespective positive or negative electrolyte port, the positive ornegative electrolyte may be evenly distributed into a respectivepositive or negative electrolyte passage via the plurality ofelectrolyte channels. In this way, the positive and negativeelectrolytes may enter and exit the internal volume of the MEA 204 viathe plurality of channels of each of the electrolyte distributioninserts 282.

As described in greater detail below with reference to FIGS. 5A-5C, eachof the positive electrolyte ports may be fluidically coupled torespective positive electrolyte passages formed between the middle andlower thermoplastic frames 272, 292 and each of the negative electrolyteports may be fluidically coupled to respective negative electrolytepassages formed between the upper and middle thermoplastic frames 262,272. In one example, upper indentations 274 a, 274 b formed in themiddle thermoplastic frame 272, in combination with an upper surface ofthe lower thermoplastic frame 292, may form the positive electrolytepassages. As further shown, upper indentations 264 a, 264 b formed inthe upper thermoplastic frame 262 may respectively receive the upperindentations 274 a, 274 b. Similarly, in one example, lower indentations284 a, 284 b formed in the middle thermoplastic frame, in combinationwith a lower surface of the upper thermoplastic frame 262, may form thenegative electrolyte passages. As further shown, lower indentations 294a, 294 b formed in the lower thermoplastic frame 292 may respectivelyreceive the lower indentations 284 a, 284 b.

Referring now to FIG. 3 , a schematic cross-sectional view 300 depictingthe MEA 204 of FIG. 2B is shown. In an exemplary embodiment, MEA 204 mayinclude the housing 210, the housing 210 at least partially enclosing aninternal volume 310 segmented at least into a positive electrodecompartment 312 and a negative electrode compartment 314. The positiveand negative electrode compartments 312, 314 may be separated by anintegrated membrane (e.g., the membrane sheet 270), which may permitionic movement between the positive and negative electrode compartments312, 314. Additionally, the MEA 204 may be fluidically coupled to anelectrolyte subsystem, which may respectively circulate positive andnegative electrolytes 322, 324 through the positive and negativeelectrode compartments 312, 314 (e.g., via pumping). In this way, theMEA 204 may be operated as a redox flow battery cell in a redox flowbattery system.

As shown in the schematic cross-sectional view 300, the upper, middle,and lower thermoplastic frames 262, 272, and 292 may be welded orotherwise adhered to one another at least along respective perimetersthereof. In one example, the upper thermoplastic frame 262 may be weldedor otherwise adhered to the middle thermoplastic frame 272 at anadherence region 330 a along respective perimeters thereof and themiddle thermoplastic frame 272 may be welded or otherwise adhered to thelower thermoplastic frame 292 at the adherence region 330 a alongrespective perimeters thereof. As further shown in the schematiccross-sectional view 300, each of the upper and lower thermoplasticframes 262, 292 may extend away in opposite directions from one anotheralong the z-axis from the respective perimeters thereof (e.g., from theadherence region 330 a). Accordingly, a positive electrode compartment312 may be formed between the upper and middle thermoplastic frames 262,272 and a negative electrode compartment 314 may be formed between themiddle and lower thermoplastic frames 272, 292. In some examples,opposite sides of the positive electrode compartment 312 may be at leastpartially formed by the upper conductive sheet 260 (e.g., welded orotherwise adhered to the upper thermoplastic frame 262 at an adherenceregion 330 b) and the membrane sheet 270 (e.g., welded or otherwiseadhered to the middle thermoplastic frame 272 at an adherence region 330c). Similarly, in some examples, opposite sides of the negativeelectrode compartment 314 may be at least partially formed by themembrane sheet 270 (e.g., welded or adhered to the middle thermoplasticframe 272 at the adherence region 330 c) and the lower conductive sheet290 (e.g., welded or adhered to the lower thermoplastic frame 292 at anadherence region 330 d).

The upper, middle, and lower thermoplastic frames 262, 272, and 292 maybe welded or otherwise adhered to one another and variously to the upperconductive sheet 260, the membrane sheet 270, and the lower conductivesheet 290 such that the positive and negative electrode compartments312, 314 are hermetically sealed thereat. Accordingly, each of a width332 a of the adherence region 330 a, a width 332 b of the adherenceregion 330 b, a width 332 c of the adherence region 330 c, and a width332 d of the adherence region 330 d may be at least a minimum widthspecified to ensure hermetic sealing during operation of the MEA 204 asa redox flow battery cell under a range of expected forces (e.g., up towhen the internal pressure of the MEA 204 may be expected to be ahighest value), compositions, and overall structural configurations. Forexample, each of the widths 332 a, 332 b, 332 c, and 332 d may be atleast 3 mm. However, other minimum width values may be selecteddepending on specific structural and compositional considerations of theMEA 204. Additionally or alternatively, each of the widths 332 a, 332 b,332 c, and 332 d may be somewhat larger than the minimum width toaccount for excess internal pressure of the MEA 204 during extremeoperating conditions (e.g., relatively high temperatures, relativelyhigh pressures associated with excess hydrogen gas production duringinitial charging of the redox flow battery system at low pH regimes andprior to rebalancing operations, etc.). For example, each of the widths332 a, 332 b, 332 c, and 332 d may be up to 1 cm.

In some examples, the upper, middle, and lower thermoplastic frames 262,272, and 292 may only be welded to one another and variously to theupper conductive sheet 260, the membrane sheet 270, and the lowerconductive sheet 290, such that no glue or other adhesive is includedbetween components of the MEA 204. In this way, costs may further bereduced during manufacturing of the MEA 204, as relatively expensiveadhesives may be eliminated.

When the MEA 204 is being operated as a redox flow battery cell, the MEA204 may expand or inflate from the base (deflated) configuration to theinflated configuration. For example, the housing 210 may expand outwardsalong the positive and negative directions of the z-axis (as indicatedby bidirectional dashed line 320), such that one or more of the upperconductive sheet 260, the membrane sheet 270, and the lower conductivesheet 290 may shift along the z-axis relative to initial position(s)thereof. In one example, the outward expansion of the housing 210 alongthe positive and negative directions of the z-axis may be substantiallyevenly distributed across portions of the upper and lower thermoplasticframes 262, 292 exposed to an external environment surrounding the MEA204 [excepting at adherence regions (e.g., 330 a), whereat the housing210 may be less flexible and correspondingly less expandable orsubstantially unexpandable]. Therewith, the internal volume 310 (e.g.,each of the positive and negative electrode compartments 312, 314) mayexpand or inflate from a first volume to a second volume responsive to afluid pressure within the internal volume 310 rising from a first fluidpressure to a second fluid pressure as the positive and negativeelectrolytes 322, 324 circulate therethrough. Contraction/expansion ofthe housing 210 along the x- and y-axes may be practically negligible.For example, though relatively small contraction along the x- and y-axesmay result as the housing is pulled taut (e.g., at the adherence region330 a) during expansion along the z-axis, such contraction may beconsidered inconsequential for practical operation of the redox flowbattery system.

Correspondingly, when the MEA 204 is not being operated as a redox flowbattery cell, the MEA 204 may contract or deflate from the inflatedconfiguration to the base (deflated) configuration. For example, thehousing 210 may contract inwards along the positive and negativedirections of the z-axis (opposite to the outward expansion indicated bythe bidirectional dashed line 320), such that one or more of the upperconductive sheet 260, the membrane sheet 270, and the lower conductivesheet 290 may shift along the z-axis relative to return to the initialposition(s) thereof. In one example, the inward contraction of thehousing 210 along the positive and negative directions of the z-axis maybe substantially evenly distributed across portions of the upper andlower thermoplastic frames 262, 292 exposed to an external environmentsurrounding the MEA 204 [excepting at adherence regions (e.g., 330 a),whereat the housing 210 may be more rigid and correspondingly lesscontractible or substantially uncontractible]. Therewith, the internalvolume 310 (e.g., each of the positive and negative electrodecompartments 312, 314) may contract or deflate from the second volume tothe first volume responsive to the fluid pressure within the internalvolume 310 dropping from the second fluid pressure to the first fluidpressure as the positive and negative electrolytes 322, 324 reduce orcease circulation therethrough.

In some examples, the internal volume 310 may be maintained below amaximum volume by physical constraints along the z-axis, e.g.,positioned so as to be in face-sharing contact when the internal volume310 is expanded to the second volume. As an example, and referring nowto FIG. 2A, the physical constraints may include a pair of adjacent MEAs204 sandwiching the MEA 204 therebetween along the z-axis (e.g., whenthe MEA 204 is positioned in a middle of the MEA stack 202, as opposedto the upper and lower ends 214 a, 214 b thereof). As another example,the physical constraints may include an adjacent MEA 204 and a pressureplate (e.g., when the MEA 204 is positioned at the upper end 214 a ofthe MEA stack 202 or the lower end 214 b of the MEA stack 202). In suchan example, an upper pressure plate may be positioned in face-sharingcontact with the MEA 204 at the upper end 214 a of the MEA stack 202 anda lower pressure plate may be positioned in face-sharing contact withthe MEA 204 at the lower end 214 b of the MEA stack 202, such that theupper and lower pressure plates may constrain expansion along the z-axisof the entire MEA stack 202. In alternative examples wherein the one ormore rebalancing cell assemblies are stacked on the MEA stack 202, theone or more rebalancing cell assemblies may be positioned between theupper pressure plate and the MEA 204 at the upper end 214 a of the MEAstack 202, or between the lower pressure plate and the MEA 204 at thelower end 214 b of the MEA stack 202.

Referring now to FIG. 3 , in some examples, electrolyte flow to the MEA204 may be controllably adjustable (e.g., based on instructions executedat the controller 88 of FIG. 1 ) such that fluid pressures appliedtherein may be withstood up to a maximum fluid pressure withoutcomponent degradation or ruptures in hermetic seals between pairs ofcomponents. In one example, a flow rate of each of the positiveelectrolyte 322 and the negative electrolyte 324 may be independentlyadjusted so as to maintain a fluid pressure difference therebetween lessthan the maximum fluid pressure. The maximum fluid pressure may beselected to prevent hydraulic crossover across the membrane sheet 270.Such hydraulic crossover may undesirably hamper electrochemicalperformance and longevity of the MEA 204. In one example, the maximumfluid pressure may be 10 kPa.

In additional or alternative examples, constricting features (e.g.,adjustable diaphragms) may be positioned along flow paths of thepositive electrolyte 322 and/or the negative electrolyte 324 todynamically restrict electrolyte flow. For example, the constrictingfeatures may passively control electrolyte flow during expansion andcontraction of the internal volume 310 (e.g., during various operationalmodes of the redox flow battery system) so as to balance and maintain apressure differential between the positive and negative electrolytes322, 324, such that the membrane sheet 270 may be selected from abroader range of compositions which may or may not include fluidcrossover mitigation features (e.g., a Nafion™ coating). In such anexample, the membrane sheet 270 may be formed absent any additionalcoating (e.g., similar to static systems, such as lead-acid batteries,which may also use non-coated membranes), which may reduce overallmanufacturing costs of the MEA 204 while substantially maintainingelectrochemical performance of the redox flow battery system.

Respective thicknesses 342, 344, and 346 of the upper, middle, and lowerthermoplastic frames 262, 272, and 292 may further be selected toprovide structural stability and strength to the housing 210. In someexamples, the thickness 344 of the middle thermoplastic frame 272 may beless than each of the thickness 342 of the upper thermoplastic frame 262and the thickness 346 of the lower thermoplastic frame 292, as the upperand lower thermoplastic frames 262, 292 may constitute a greaterproportion of external surfaces of the housing 210 (and as such, theupper and lower thermoplastic frames 262, 292 may be subjected togreater aggregate force than the middle thermoplastic frame 272 duringoperation of the MEA 204 as a redox flow battery cell). For example, thethickness 344 of the middle thermoplastic frame 272 may be at least 0.07mm, while each of the thickness 342 of the upper thermoplastic frame 262and the thickness 346 of the lower thermoplastic frame 292 may be atleast 1 mm. Accordingly, in such examples, the middle thermoplasticframe 272 may be more flexible than the upper and lower thermoplasticframes 262, 292.

Various components may be positioned within the positive electrodecompartment 312 to facilitate the redox reaction of equation (2). Forexample, and as shown in the schematic cross-sectional view 300, apositive electrode 302 may be positioned in the positive electrolyte 322between the upper conductive sheet 260 and the membrane sheet 270. Insome examples, the positive electrode 302 may be composed of a felt,such as a carbon felt, which may assist in oxidation of Fe²⁺ when incontact with the positive electrolyte 322 during charging of the redoxflow battery system and reduction of Fe³⁺ when in contact with thepositive electrolyte 322 during discharging of the redox flow batterysystem.

Similarly, various components may be positioned within the negativeelectrode compartment 314 to facilitate the plating reaction of equation(1). For example, and as shown in the schematic cross-sectional view300, a negative electrode spacer 304 may be positioned in the negativeelectrolyte 324 between the membrane sheet 270 and the lower conductivesheet 290. In some examples, the negative electrode spacer 304 may becomposed of a non-conductive mesh, such as a plastic mesh, which maydefine flow channels (e.g., along a surface of a negative electrode, notshown at FIG. 3 ) for distribution of the negative electrolyte 324 andplating of Fe²⁺ therefrom and may further function as a separator toprevent the membrane sheet 270 from coming within a threshold proximityof the lower conductive sheet 290 (which may result in electricalshorting, in certain examples). In some examples, a face of the lowerconductive sheet 290 facing in a positive direction along the z-axis(e.g., in fluidic contact with the negative electrolyte 324 duringoperation of the redox flow battery system) may be textured to increasea surface area at which chemical reactions may occur (e.g., the platingreaction of equation (1)).

In additional or alternative examples, a non-conductive felt strip maybe positioned in the negative electrode compartment 314, e.g., at oradjacent to an inlet of the negative electrolyte 324, for furtherimproving flow distribution of the negative electrolyte 324 via creationof relatively small amounts of fluid backpressure (and thereby apressure drop) in the negative electrolyte 324 entering into thenegative electrode compartment 314. In such examples, a length of thenon-conductive felt strip along the y-axis may be substantiallyequivalent to a length of an active area within the negative electrodecompartment 314 for Fe⁰ plating along the y-axis (e.g., a length of aportion of the lower conductive sheet 290 in fluidic contact with thenegative electrolyte 324), while a width of the non-conductive feltstrip along the x-axis may be relatively small (e.g., ˜1 cm).

The positive and negative electrode compartments 312, 314 may berespectively filled with the positive and negative electrolytes 322,324, immersing the various components therein, such as the positiveelectrode 302 and the negative electrode spacer 304, respectively. In anexemplary embodiment, during operation of the MEA 204 as a redox flowbattery cell, the positive electrolyte 322 may circulate through thepositive electrode compartment 312 such that an available volume of thepositive electrode compartment 312 is substantially entirely filled bythe positive electrolyte 322. Accordingly, fluid pressure may increasein the positive electrode compartment 312 during operation of the MEA204 as a redox flow battery cell, thereby expanding or inflating thehousing 210. Similarly, during operation of the MEA 204 as a redox flowbattery cell, the negative electrolyte 324 may circulate through thenegative electrode compartment 314 such that an available volume of thenegative electrode compartment 314 is substantially entirely filled bythe negative electrolyte 324. Accordingly, fluid pressure may increasein the negative electrode compartment 314 during operation of the MEA204 as a redox flow battery cell, thereby expanding or inflating thehousing 210.

Referring now to FIGS. 4A-4C, perspective cross-sectional views 400 and440 and a schematic cross-sectional view 480 depicting exemplaryconfigurations of the electrolyte distribution insert 282 arerespectively shown (the schematic cross-sectional view 480 of FIG. 4Cbeing indicated by a segmenting plane defined by a cutline 4C-4C in theperspective cross-sectional view 440 of FIG. 4B). In an exemplaryembodiment, the electrolyte distribution insert 282 may be formed as amolded plastic ring including a plurality of electrolyte channels 408disposed between a plurality of electrolyte channel partitions 406. Theelectrolyte distribution insert 282 may be positioned within anelectrolyte port of the MEA 204, such as the first negative electrolyteport 208 a formed from the first negative electrolyte port openings 268a, 278 a, and 298 a disposed in the upper, middle, and lowerthermoplastic frames 262, 272, and 292, respectively, (as shown in theperspective cross-sectional view 400 of FIG. 4A) or the first positiveelectrolyte port 206 a formed from the first positive electrolyte portopenings 266 a, 276 a, and 296 a disposed in the upper, middle, andlower thermoplastic frames 262, 272, and 292, respectively (as shown inthe perspective cross-sectional view 440 of FIG. 4B). Accordingly, theelectrolyte distribution insert 282 may form a fluidic coupling betweena given electrolyte port and an electrolyte passage formed betweenthermoplastic sheets of the housing 210.

As an example, the electrolyte distribution insert 282 may fluidicallycouple the first negative electrolyte port 208 a to a negativeelectrolyte passage 414 via the plurality of electrolyte channels 408.As another example, the electrolyte distribution insert 282 mayfluidically couple the first positive electrolyte port 206 a to apositive electrolyte passage 412 via the plurality of electrolytechannels 408. As discussed in detail below, the electrolyte distributioninserts 282 may mechanically interlock with one another, such that acommon fluid manifold (e.g., the first negative electrolyte manifold 218a of FIG. 2A, the first positive electrolyte manifold 216 a of FIG. 2A,etc.) may be formed by stacking, e.g., along the z-axis, andmechanically interlocking multiple MEAs 204 via the electrolytedistribution inserts 282. In this way, positive and negativeelectrolytes (e.g., positive and negative electrolytes 322, 324 of FIG.3 ; not shown at FIGS. 4A-4C) may be pumped into and circulated througha stack of MEAs 204 being operated as redox flow battery cells in aredox flow battery system via common fluid manifolds formed from one ormore electrolyte distribution inserts 282 included in each MEA 204.

The electrolyte distribution insert 282 may include upper and lowerplates 402, 404 which respectively form upper and lower surfaces of theplurality of electrolyte channels 408. In some examples, the electrolytedistribution insert 282 may be molded such that the upper and lowerplates 402, 404 have larger respective inner diameters 403, 405 than theelectrolyte port in which the electrolyte distribution insert 282 ispositioned. Furthermore, the inner diameter 403 of the upper plate 402may be larger than the inner diameter 405 of the lower plate, as shownin FIGS. 4A-4B. Accordingly, in some examples, upon positioning of theelectrolyte distribution insert 282 between a given pair ofthermoplastic sheets of the housing 210 and welding the given pair ofthermoplastic sheets to one another, the electrolyte distribution insert282 may be sealingly fixed in position at a given electrolyte port.

In some examples, and as shown in the perspective cross-sectional views400 and 440 of FIGS. 4A and 4B, respectively, the electrolytedistribution insert 282 may be welded or otherwise adhered to thehousing 210 at adherence regions 430. In one example (e.g., when theelectrolyte distribution insert 282 is positioned at one of the negativeelectrolyte ports, such as the first negative electrolyte port 208 a),the upper plate 402 may be welded or otherwise adhered to the upperthermoplastic frame 262 at one of the adherence regions 430 extendingaround a circumference of the electrolyte distribution insert 282 so asto form a hermetic sealing thereat and the lower plate 404 may be weldedor otherwise adhered to the middle thermoplastic frame 272 at anotherone of the adherence regions 430 extending around the circumference ofthe electrolyte distribution insert 282 so as to form a hermetic sealingthereat. Similarly, in one example (e.g., when the electrolytedistribution insert 282 is positioned at one of the positive electrolyteports, such as the first positive electrolyte port 206 a), the upperplate 402 may be welded or otherwise adhered to the middle thermoplasticframe 272 at one of the adherence regions 430 extending around thecircumference of the electrolyte distribution insert 282 so as to form ahermetic sealing thereat and the lower plate 404 may be welded orotherwise adhered to the lower thermoplastic frame 292 at another one ofthe adherence regions 430 extending around the circumference of theelectrolyte distribution insert 282 so as to form a hermetic sealingthereat.

Accordingly, a width 432 (as shown in FIG. 4A) of each of the adherenceregions 430 may be at least a minimum width specified to ensure hermeticsealing during operation of the MEA 204 as a redox flow battery cell(e.g., when the internal pressure of the MEA 204 may be expected to be ahighest value). For example, each width 432 may be at least 3 mm.However, other minimum width values may be selected depending onspecific structural and compositional considerations of the MEA 204.Additionally or alternatively, each width 432 may be somewhat largerthan the minimum width to account for excess internal pressure of theMEA 204 during extreme operating conditions (e.g., relatively hightemperatures, relatively high pressures associated with excess hydrogengas production during initial charging of the redox flow battery systemat low pH regimes and prior to rebalancing operations, etc.). Forexample, each width 432 may be up to 1 cm.

The plurality of electrolyte channel partitions 406 may be disposedbetween the upper and lower plates 402, 404 and substantially evenlyspaced around a circumference of the electrolyte distribution insert 282so as to form the plurality of electrolyte channels 408. Accordingly, anumber of the plurality of electrolyte channel partitions 406 may beequal to a number of the plurality of electrolyte channels 408. Thoughthe electrolyte distribution insert 282 is shown in the views 400, 440,and 480 as having 12 electrolyte channel partitions 406 and 12electrolyte channels 408, the number of the plurality of electrolytechannel partitions 406 and the number of the plurality of electrolytechannels 408 included in the electrolyte distribution insert 282 may notbe particularly limited and greater or fewer electrolyte channelpartitions 406 and electrolyte channels 408 may be included in theelectrolyte distribution insert 282 according to performance demands ofa particular application.

As one example, the number of the plurality of electrolyte channelpartitions 406 and the number of the plurality of electrolyte channels408 may be selected so as to permit as much electrolyte flow as possiblewhile retaining structural integrity of the electrolyte distributioninsert 282. Dimensions of each of the plurality of electrolyte channelpartitions 406 and the plurality of electrolyte channels 408 may dependon similar considerations, e.g., balancing electrolyte flow withstructural integrity. As an example, a width 482, as shown in FIG. 4C,of each of the plurality of electrolyte channel partitions 406 (e.g.,approximately parallel to the circumference of the electrolytedistribution insert 282) may have a lower limit specific to acomposition of the electrolyte distribution insert 282 at whichstructural integrity may be reliably maintained. As another example, awidth 484, as shown in FIG. 4C, of each of the plurality of electrolytechannels 408 (e.g., approximately parallel to the circumference of theelectrolyte distribution insert 282) may have a lower limit such thatclogging of the plurality of electrolyte channels 408 may be mitigated.

The MEA 204 may receive the positive and negative electrolytes viaelectrolyte ports fluidically coupled to an electrolyte subsystem of theredox flow battery system. By sealingly fixing the electrolytedistribution inserts 282 at respective electrolyte ports, in someexamples, the positive and negative electrolytes may only enter the MEA204 via the plurality of electrolyte channels 408. As an example, and asindicated by an arrow 410 in FIG. 4A, a negative electrolyte may flowfrom the first negative electrolyte port 208 a and into the negativeelectrolyte passage 414 via the plurality of electrolyte channels 408.As another example, and as indicated by an arrow 450 in FIG. 4B, apositive electrolyte may flow from the first positive electrolyte port206 a and into the positive electrolyte passage 412 via the plurality ofelectrolyte channels 408.

The electrolyte distribution insert 282 may include upper and lowerflanges 416, 418 for mechanical interlocking of the electrolytedistribution insert 282 with other electrolyte distribution inserts 282.In one example, the upper and lower flanges 416, 418 of a pair ofelectrolyte distribution inserts 282 may matingly engage with oneanother, e.g., shaped to mate with one another, at an interlockinginterface or region 452, as shown in FIG. 4B, such that the pair ofelectrolyte distribution inserts 282 mechanically interlock with oneanother. As an example, the lower flange 418 of one electrolytedistribution insert 282 of the pair of electrolyte distribution inserts282 may matingly engage the upper flange 416 of the other electrolytedistribution insert 282 of the pair of electrolyte distribution inserts282 at the interlocking interface or region 452. In other words, thelower flange 418 of a first electrolyte distribution insert may slidedownwards, along the z-axis, into the upper flange 416 of a secondelectrolyte distribution insert 282 positioned directly below the firstelectrolyte distribution insert, and fit tightly within the innerdiameter 403 of the upper flange 416.

As another example, and as shown in the perspective cross-sectional view440 of FIG. 4B, the upper flange 416 of one electrolyte distributioninsert 282 of the pair of electrolyte distribution inserts 282 maymatingly engage the lower flange 418 of the other electrolytedistribution insert 282 of the pair of electrolyte distribution inserts282 at the interlocking interface or region 452. It should beappreciated that, in some examples, the electrolyte distribution insert282 shown mechanically interlocked with the electrolyte distributioninsert 282 positioned in the first positive electrolyte port 206 a maybe positioned in an electrolyte port of another MEA 204 (not shown atFIG. 4B). In this way, the electrolyte distribution inserts 282 may beconfigured to mechanically interlock via the upper and lower flanges416, 418, thereby mechanically and fluidically coupling an adjacent pairof MEAs 204 (e.g., in an MEA stack, such as the MEA stack 202 of FIG.2A).

Referring now to FIGS. 5A-5C, perspective cross-sectional views 500 and540 of one of the MEAs 204 of FIG. 2A, and a schematic top view 580depicting exemplary configurations of the positive and negativeelectrolyte passages 412, 414 are respectively shown. The perspectivecross-sectional view 540 of FIG. 5B is a detailed view of region 501indicated in FIG. 5A. In an exemplary embodiment, the positive andnegative electrolyte passages 412, 414 may be formed between adjacentthermoplastic sheets of the housing 210, fluidically coupling theelectrolyte ports 206 a, 206 b, 208 a, and 208 b to the positive andnegative electrode compartments (e.g., the positive and negativeelectrode compartments 312, 314 of FIG. 3 ; not shown at FIGS. 5A-5C)via an anti-shunting flow path.

For example, as shown in FIG. 5B, the positive electrolyte passages 412may be formed between an upper surface 297 a of the lower thermoplasticframe 292 and upper indentations (e.g., the upper indentation 274 a) ofthe middle thermoplastic frame 272, and the negative electrolytepassages 414 may be formed between a lower surface 267 b of the upperthermoplastic frame 262 and lower indentations (e.g., the lowerindentation 284 a) of the middle thermoplastic frame 272. Accordingly,the positive electrolyte passages 412 may be extruded outwards from anupper surface 267 a of the upper thermoplastic frame 262 (e.g., along apositive direction of the z-axis) and the negative electrolyte passages414 may be extruded outwards from a lower surface 297 b of the lowerthermoplastic frame 292 (e.g., along a negative direction of thez-axis).

The positive and negative electrolyte passages 412, 414 may behermetically sealed along interfaces of the thermoplastic sheets formingthe positive and negative electrolyte passages 412, 414 at adherenceregions 530, such that the positive and negative electrolytes (e.g., thepositive and negative electrolytes 322, 324 of FIG. 3 ; not shown atFIGS. 5A-5C) may only enter and exit the positive and negativeelectrolyte passages 412, 414 via the electrolyte ports 206 a, 206 b,208 a, and 208 b and the positive and negative electrode compartments.In this way, when the MEA 204 is operated as a redox flow battery cellin a redox flow battery system, the positive electrolyte may enter theMEA 204 via one of the first and second positive electrolyte ports 206 aand 206 b, flow along one of the positive electrolyte passages 412 tothe positive electrode compartment, and flow therefrom along the otherone of the positive electrolyte passages 412 to exit the MEA 204 via theother one of the first and second positive electrolyte ports 206 a and206 b. Similarly, when the MEA 204 is operated as a redox flow batterycell in the redox flow battery system, the negative electrolyte mayenter the MEA 204 via one of the first and second negative electrolyteports 208 a and 208 b, flow along one of the negative electrolytepassages 414 to the negative electrode compartment, and flow therefromalong the other one of the negative electrolyte passages 414 to exit theMEA 204 via the other one of the first and second negative electrolyteports 208 a and 208 b.

Owing to the modularity of the MEAs 204 (e.g., facile mechanical/fluidiccoupling/decoupling from one another), aspects of an overallconfiguration of the positive and negative electrolyte passages 412, 414(e.g., shapes of the anti-shunting flow paths, numbers of the positiveand negative electrolyte passages 412, 414 included in the MEA 204,etc.) may be readily modified by removing and replacing MEAs 204 fromthe redox flow battery system (e.g., from an MEA stack, such as the MEAstack 202 of FIG. 2A) as desired. For example, though the positive andnegative electrolyte passages 412, 414 are depicted in the views 500,540, and 580 as having anti-shunting flow paths with two bends, otherflow path configurations, including shorter, more direct flow paths(e.g., having one bend or no bends) or longer, less direct flow paths(e.g., having more than two bends), may be selected according toperformance demands of a particular application.

For instance, decreasing a number of bends in each of the anti-shuntingflow paths may correspondingly decrease backpressure of the positive andnegative electrolytes (e.g., the positive and negative electrolytes 322,324; not shown at FIGS. 5A-5C), thereby reducing parasitic powerconsumption at electrolyte pumps (e.g., the negative and positiveelectrolyte pumps 30, 32 of FIG. 1 ; not shown at FIGS. 5A-5C) of theredox flow battery system. However, the number of bends in each of theanti-shunting flow paths may be selected sufficiently high to realize aminimum length of a given anti-shunting flow path under packagingconstraints of a given MEA 204. Such shunt protection considerations maydiminish with a decreasing overall number of the MEAs 204 included inthe MEA stack 202, such that the minimum length of each of theanti-shunting flow paths may be correspondingly reduced and fewer or nobends may be included therein in certain examples.

Further, though the MEA 204 is shown in the views 500, 540, and 580 ashaving two positive electrolyte passages 412 and two negativeelectrolyte passages 414, the number of the positive electrolytepassages 412 and the number of negative electrolyte passages 414 is notparticularly limited, as long as at least two positive electrolytepassages 412 are provided for respective entry and exit of the positiveelectrolyte and at least two negative electrolyte passages 414 areprovided for respective entry and exit of the negative electrolyte, andgreater positive and negative electrolyte passages 412, 414 may beincluded in the MEA 204 according to performance demands of a particularapplication.

As shown in the perspective cross-sectional views 500 and 540 of FIGS.5A and 5B, respectively, the upper, middle, and lower thermoplasticframes 262, 272, and 292 may be welded or otherwise adhered to oneanother at least at sealing edges of the positive and negativeelectrolyte passages 412, 414 extending in a plane defined by the x- andy-axes. In one example, the upper thermoplastic frame 262 may be weldedor otherwise adhered to the middle thermoplastic frame 272 and themiddle thermoplastic frame 272 may be welded or otherwise adhered to thelower thermoplastic frame 292 at the adherence regions 530 disposedalong the sealing edges of the positive and negative electrolytepassages 412, 414 such that the positive and negative electrolytepassages 412, 414 may be hermetically sealed thereat. Accordingly, awidth 532 of each of the adherence regions 530 may be at least a minimumwidth specified to ensure hermetic sealing during operation of the MEA204 as a redox flow battery cell (e.g., when the internal pressure ofthe MEA 204 may be expected to be a highest value). For example, eachwidth 532 may be at least 3 mm. However, other minimum width values maybe selected depending on specific structural and compositionalconsiderations of the MEA 204.

Additionally or alternatively, each width 532 may be somewhat largerthan the minimum width to account for excess internal pressure of theMEA 204 during extreme operating conditions (e.g., relatively hightemperatures, relatively high pressures associated with excess hydrogengas production during initial charging of the redox flow battery systemat low pH regimes and prior to rebalancing operations, etc.). Forexample, each width 532 may be up to 1 cm. In additional or alternativeexamples, each width 532 may be less than each of the widths 332 a, 332b, 332 c, 332 d of FIG. 3 , as lower overall forces may be sustained atthe adherence regions 530 relative to the adherence regions 330 a, 330b, 330 c, 330 d of FIG. 3 during expansion/contraction of the MEA 204.

In some examples, the positive and negative electrolyte passages 412,414 may each have distinct sections fluidically coupled to one another,within the respective electrolyte passage, in series. For example, asshown in FIG. 5C, the positive electrolyte passages 412 may be formedfrom a sequential fluidic coupling of a plenum 564 a (which may furtherbe fluidically coupled to one of the first and second positiveelectrolyte ports 206 a, 206 b), a first lengthwise passage section 564b, a first passage bend 564 c, a second lengthwise passage section 564d, and a second passage bend 564 e (which may further be fluidicallycoupled to the positive electrode compartment, the second passage bend564 e being partially overlapped by the upper conductive sheet 260).Similarly, the negative electrolyte passages 414 may be formed from asequential fluidic coupling of a plenum 594 a (which may further befluidically coupled to one of the first and second negative electrolyteports 208 a, 208 b), a first lengthwise passage section 594 b, a firstpassage bend 594 c, a second lengthwise passage section 594 d, and asecond passage bend 594 e [which may further be fluidically coupled tothe negative electrode compartment, the second passage bend 594 e beingat least partially overlapped by the lower conductive sheet 290 of FIG.2B (not shown at FIG. 5C)]. The schematic top view 580 of FIG. 5Cdepicts each of the distinct sections of the positive electrolytepassages 412 as encompassed by the upper indentations 264 a, 264 b(depicted in solid lines) and each of the distinct sections of thenegative electrolyte passages 414 as encompassed by the lowerindentations 294 a, 294 b (depicted in dashed lines, being on anopposite side of the MEA 204 from the features visible in the schematictop view 580).

In examples wherein the positive and negative electrolyte passages 412,414 are formed in the distinct sections, the positive and negativeelectrolytes may accordingly flow through the distinct sections insequence (e.g., from the plenum 564 a or 594 a to the second passagebend 564 e or 594 e) or in reverse sequence (e.g., from the secondpassage bend 564 e or 594 e to the plenum 564 a or 594 a). In oneexample, and as indicated by an arrow 550 in FIG. 5A, the positiveelectrolyte may enter the MEA 204 via the first positive electrolyteport 206 a and flow therefrom through one of the positive electrolytepassages 412, sequentially via the plenum 564 a, the first lengthwisepassage section 564 b, the first passage bend 564 c, the secondlengthwise passage section 564 d, and the second passage bend 564 e (asshown in FIG. 5C), to the positive electrode compartment. Further, thepositive electrolyte may flow from the positive electrode compartment tothe second positive electrolyte port 206 b through the other one of thepositive electrolyte passages 412, sequentially via the second passagebend 564 e, the second lengthwise passage section 564 d, the firstpassage bend 564 c, the first lengthwise passage section 564 b, and theplenum 564 a, to exit the MEA 204.

Similarly, in one example, and as indicated by an arrow 510 in FIG. 5A,the negative electrolyte may enter the MEA 204 via the first negativeelectrolyte port 208 a and flow therefrom through one of the negativeelectrolyte passages 414, sequentially via the plenum 594 a, the firstlengthwise passage section 594 b, the first passage bend 594 c, thesecond lengthwise passage section 594 d, and the second passage bend 594e (as shown in FIG. 5C), to the negative electrode compartment. Further,the negative electrolyte may flow from the negative electrodecompartment to the second negative electrolyte port 208 b through theother one of the negative electrolyte passages 414, sequentially via thesecond passage bend 594 e, the second lengthwise passage section 594 d,the first passage bend 594 c, the first lengthwise passage section 594b, and the plenum 594 a, to exit the MEA 204. In other examples, thepositive and negative electrolytes may flow through the MEA 204 in acrosswise configuration (e.g., the positive electrolyte may enter theMEA 204 via the first positive electrolyte port 206 a and exit the MEA204 via the second positive electrolyte port 206 b and the negativeelectrolyte may enter the MEA 204 via the second negative electrolyteport 208 b and exit the MEA 204 via the first negative electrolyte port208 a, and vice versa).

Referring now to FIG. 6 , a flow chart of an example of a method 600 forassembling an MEA stack via roll-to-roll processing, operating the MEAstack as a redox flow battery, and testing and diagnosing the MEA stackis shown. Individual MEAs of the MEA stack may be assembled viaroll-to-roll processing, which may lessen or mitigate issues ascribed tomolding-based electrode assembly manufacturing processes. For example,relative to such molding-based electrode assembly manufacturingprocesses, roll-to-roll processing may decrease tolerance stacking ofthe MEAs, reduce an overall cost and floor space, and facilitatereplacement of degraded, defective, outmoded, or outsized manufacturingcomponents (thereby permitting manufacturing of MEAs for larger formatbatteries, which may be restricted by molding press sizing limits in themolding-based processes). Further, by employing roll-to-roll processing,relatively flexible materials, such as extruded thermoplastic sheets,may be employed in forming a housing of each of the MEAs, which mayallow for easier diagnosis and recycling of degraded components includedin each of the MEAs. In this way, an overall flexibility of each of amanufacturing process of the MEAs and operation and testing of thefinally-formed MEAs may be increased by employing roll-to-rollprocessing instead of molding-based processes.

In some examples, the MEA stack may be implemented in a redox flowbattery system and fluidically coupled to an electrolyte subsystemtherein, such that positive and negative electrolytes may be circulatedthrough the MEA stack during operation thereof as a redox flow battery.In an exemplary embodiment, the redox flow battery system may be theredox flow battery system 10 of FIG. 1 and the MEA stack may be the MEAstack 202 of FIG. 2A. Accordingly, method 600 may be considered withreference to the embodiments of FIGS. 1-5C (though it will be understoodthat similar methods may be applied to the aforementioned or otherembodiments without departing from the scope of the present disclosure).For example, with method 600, at least some steps or portions of steps(e.g., involving operation and testing of the MEA stack as a redox flowbattery) may be carried out via the controller 88 of FIG. 1 , and may bestored as executable instructions at a non-transitory storage medium(e.g., memory) communicably coupled to the controller 88. Furthercomponents described with reference to FIG. 6 may be examples ofcorresponding components of FIGS. 1-5C.

At 602, method 600 includes sequentially stacking layers to form an MEAvia roll-to-roll processing (as used with reference to forming the MEAvia roll-to-roll processing, “sequentially stacking layers” may refer topositioning and/or welding or otherwise adhering various layers, sheets,or components of the MEA). In one example, an MEA housing may be formedby welding or otherwise adhering a plurality of extruded thermoplasticsheets or frames. Conductive sheets may be welded or otherwise adheredto an exterior surface of the MEA housing so as to be fluidically andelectronically coupled to an interior of the MEA. A membrane sheet maybe welded or otherwise adhered to an interior surface of the MEA housingso as to bisect an internal volume of the MEA into positive and negativeelectrode compartments while permitting ionic conduction therebetween.Various electrode components, such as a positive electrode and anegative electrode spacer, may be positioned within the positive andnegative electrode compartments. Further details of such roll-to-rollprocessing are described below with reference to method 700 of FIG. 7 .As one embodiment, method 700 of FIG. 7 may partially or whollysubstitute 602. However, it will be appreciated that method 700 of FIG.7 constitutes one exemplary embodiment of roll-to-roll processing ofMEAs and that additional or alternative roll-to-roll processing methodsmay be implemented within the scope of the present disclosure.

At 604, method 600 includes conducting a pneumatic pressure test on theMEA. The pneumatic pressure test may include diagnosing whether or notany leak is present in hermetic seals of the MEA or whether the hermeticseals have relatively weaker sealing regions which rupture during thepneumatic pressure test.

At 606, method 600 includes determining whether the MEA passed thepneumatic pressure test (e.g., whether or not a pass flag was generatedfor the MEA). If the MEA is determined to be degraded (e.g., if the MEAdoes not pass the pneumatic pressure test due to presence or formationof one or more leaks), method 600 proceeds to 608, where method 600includes resealing (e.g., spot welding or otherwise re-adhering the oneor more leaks) or replacing the degraded MEA with an MEA whichsuccessfully passes the pneumatic pressure test (e.g., formed, tested,and diagnosed at 602, 604, and 606). If the MEA is determined to benon-degraded (e.g., if the MEA passes the pneumatic pressure test and isdetermined to be hermetically sealed at adherence regions thereof) or ifthe degraded MEA is resealed or replaced at 608, method 600 proceeds to610, where method 600 includes adding the MEA to the MEA stack.

In some examples, each respective MEA of the MEA stack may includechanneled electrolyte distribution inserts, the channeled electrolytedistribution inserts mechanically interlocking the respective MEA withan adjacent MEA (e.g., at an upper or lower end of the MEA stack). Insuch examples, the channeled electrolyte distribution inserts may alsofluidically couple the interior of the respective MEA to fluid manifoldsof the MEA stack, such that positive and negative electrolytes may berespectively admitted into the interior of the respective MEA from oneor more of the fluid manifolds via channels of the channeled electrolytedistribution inserts.

At 612, method 600 includes determining whether a most recentlyassembled and added MEA (e.g., the MEA assembled at 602 and added to theMEA stack at 610 after being tested at 604 and diagnosed at 606) is thelast MEA to be added to the MEA stack. If the most recently assembledand added MEA is not the last MEA to be added to the MEA stack, method600 returns to 602, where method 600 includes assembling another MEA viaroll-to-roll processing.

If the most recently assembled and added MEA is the last MEA to be addedto the MEA stack, assembly of the MEA stack (including 602, 604, 606,608, 610, and 612) via roll-to-roll processing may be consideredcomplete and method 600 proceeds to 614, where method 600 includescoupling (e.g., electrically coupling) testing probes to one or moreMEAs of the MEA stack while the MEA stack is not being operated as aredox flow battery. As one example, the testing probes may be insertedin between adjacent pairs of MEAs of the MEA stack and/or at an upper orlower end of the MEA stack when MEAs included in the MEA stack aresufficiently contracted or deflated (e.g., prior to the MEA stack beingoperated as a redox flow battery). In one example, for a given MEA beingtested, one testing probe may be positioned so as to electrically coupleto one side of the given MEA and another testing probe may be positionedso as to electrically couple to an opposite side of the given MEA.

At 616, method 600 includes operating the MEA stack as a redox flowbattery while respectively pumping the positive and negativeelectrolytes into the MEA stack through one or more of the fluidmanifolds. In some examples, the fluid manifolds may fluidically coupleeach MEA of the MEA stack to one another, such that the positive andnegative electrolytes may be distributed to each MEA of the MEA stackduring operation of the MEA stack as a redox flow battery. In additionalor alternative examples, during operation of the MEA stack as a redoxflow battery, the positive and negative electrolytes may be admittedinto, and expelled from, the interior of each MEA of the MEA stack fromthe fluid manifolds via channels of the channeled electrolytedistribution inserts (where some channeled electrolyte distributioninserts admit the positive and negative electrolytes into the interiorsof the MEAs of the MEA stack and other channeled electrolytedistribution inserts expel the positive and negative electrolytes fromthe interiors of the MEAs of the MEA stack).

Because of the flexibility of the extruded thermoplastic sheets orframes forming the housing of the MEAs included in the MEA stack, theMEA stack may be expanded or inflated when the MEA stack is beingoperated as a redox flow battery and contracted or deflated when the MEAstack is not being operated as a redox flow battery. In one example, theMEA stack may be expanded to a first volume via a first fluid pressure(e.g., from the positive and negative electrolytes being pumped throughthe MEA stack) while the MEA stack is being operated as a redox flowbattery and the MEA stack may be contracted to a second volume via asecond fluid pressure (e.g., from the positive and negative electrolyteshaving a relatively low flow rate or being substantially stationary)while the MEA stack is not being operated as a redox flow battery, thefirst volume being greater than the second volume and the first fluidpressure being greater than the second fluid pressure. In this way, theextruded thermoplastic sheets or frames may provide increasedflexibility during operation of the MEA stack as a redox flow battery bypermitting relatively large volume changes in the interiors of the MEAsincluded in the MEA stack. Further, by increasing a volume of the MEAstack, adjacent MEAs of the MEA stack may come into physical contactwith one another, such that an electrical contact resistance between theadjacent MEAs (e.g., between conductive sheets of the adjacent MEAsforced into face-sharing contact with one another) may be lower whilethe MEA stack is being operated as a redox flow battery than while theMEA stack is not being operated as a redox flow battery.

At 618, method 600 includes determining whether one or more testingconditions have been met. As one example, the one or more testingconditions may include a predetermined cumulative duration of operationof the MEA stack as a redox flow battery having elapsed sincemanufacture or most recent testing. If the one or more testingconditions have not been met, method 600 proceeds to 620, where method600 includes continuing to operate the MEA stack as a redox flowbattery. Accordingly, method 600 returns to 616.

If the one or more testing conditions have been met, method 600 proceedsto 622, where method 600 includes conducting voltage testing of thegiven MEA via testing probes. The voltage testing may include testing apotential difference across the given MEA with the testing probes whilethe MEA stack is being charged and discharged during operation thereofas a redox flow battery. The voltage testing may continue to beconducted until sufficient data is collected for the given MEA togenerate a diagnosis therefor.

If testing is determined to be complete, method 600 may proceed to 624,where method 600 includes determining whether all MEAs of the MEA stackpassed the voltage testing (e.g., whether or not a pass flag wasgenerated for all MEAs of the MEA stack). If at least one of the MEAs ofthe MEA stack is determined to be degraded (e.g., if at least one of theMEAs of the MEA stack does not pass testing), method 600 proceeds to626, where method 600 includes ceasing operating the MEA stack as aredox flow battery to reduce a fluid pressure of the MEA stack. In oneexample, a fluid pressure of the MEA stack may reduce from the firstfluid pressure to the second fluid pressure and a volume of the MEAstack may concomitantly reduce from the first volume to the secondvolume as the MEAs included in the MEA stack contract or deflate. Oncethe MEAs included in the MEA stack are sufficiently contracted ordeflated (e.g., while the MEA stack is not being operated as a redoxflow battery), at 628, method 600 includes replacing the at least onedegraded MEA. For example, the at least one degraded MEA may berespectively replaced with at least one MEA that successfully passes thevoltage testing.

If all of the MEAs of the MEA stack are determined to be non-degraded(e.g., if all of the MEAs of the MEA stack pass voltage testing) or ifsome of the MEAs of the MEA stack are determined to be non-degraded andthe at least one remaining (degraded) MEAs of the MEA stack have beenreplaced at 628, method 600 proceeds to 630, where method 600 includesresuming operation of the MEA stack as a redox flow battery.

Referring now to FIG. 7 , a flow chart of an example of a method 700 forassembling an MEA via roll-to-roll processing is shown. In one example,the MEA may be assembled by variously welding or otherwise adhering aplurality of extruded thermoplastic sheets or frames, two conductivesheets, and a membrane sheet to one another, positioning positive andnegative electrode components to correspond to finally-formed positiveand negative electrode compartments, respectively, and cutting theplurality of extruded thermoplastic sheets or frames from a (final)roll. In this way, manufacturing tolerances may be more closely adheredto and manufacturing adjustments may be made more easily than inmolding-based electrode assembly manufacturing processes.

In some examples, the MEA may be added to an MEA stack to be implementedin a redox flow battery system. In an exemplary embodiment, the redoxflow battery system may be the redox flow battery system 10 of FIG. 1and the MEA may be the MEA 204 of FIGS. 2A-4B and 5A-5C. Accordingly,method 700 may be considered with reference to the embodiments of FIGS.1-5C (though it will be understood that similar methods may be appliedto the aforementioned or other embodiments without departing from thescope of the present disclosure).

At 702, method 700 includes welding or otherwise adhering a firstconductive sheet (e.g., a carbon fiber sheet) to a first thermoplasticsheet. Upon welding or otherwise adhering the first conductive sheet tothe first thermoplastic sheet, the first conductive sheet mayhermetically seal an opening disposed within the first thermoplasticsheet.

At 704, method 700 includes positioning a negative electrode spacer(e.g., a non-conductive mesh spacer) on the first conductive sheet. Insome examples, further negative electrode components may be stacked withthe negative electrode spacer on the first conductive sheet, such as anon-conductive felt strip (e.g., positioned so as to increase a pressuredrop and improve a flow distribution of a negative electrolyte in theMEA as finally formed).

At 706, method 700 includes welding or otherwise adhering a secondthermoplastic sheet to the first thermoplastic sheet. The secondthermoplastic sheet may be welded or otherwise adhered to the firstthermoplastic sheet such that an adherence region of the firstconductive sheet and the first thermoplastic sheet is circumscribed byan adherence region of the first and second thermoplastic sheets.

At 708, method 700 includes welding or otherwise adhering a membranesheet to the second thermoplastic sheet. Upon welding or otherwiseadhering the membrane sheet to the second thermoplastic sheet, themembrane sheet may hermetically seal an opening disposed within thesecond thermoplastic sheet such that a negative electrode compartmenthousing the negative electrode spacer (and any further negativeelectrode components) is formed.

At 710, method 700 includes positioning a positive electrode (e.g., acarbon felt electrode) on the membrane sheet.

At 712, method 700 includes welding or otherwise adhering a thirdthermoplastic sheet to the second thermoplastic sheet. As one example,the third thermoplastic sheet may be welded or otherwise adhered to thesecond thermoplastic sheet such that an orthographic projection of theadherence region of the first and second thermoplastic sheets overlapsan orthographic projection of an adherence region of the second andthird thermoplastic sheets.

At 714, method 700 includes welding or otherwise adhering a secondconductive sheet (e.g., a carbon fiber sheet) to the third thermoplasticsheet. Upon welding or otherwise adhering the second conductive sheet tothe third thermoplastic sheet, the second conductive sheet mayhermetically seal an opening disposed within the third thermoplasticsheet. Further, the second conductive sheet may be welded or otherwiseadhered to the third thermoplastic sheet such that an adherence regionof the second conductive sheet and the third thermoplastic sheet iscircumscribed by the adherence region of the second and thirdthermoplastic sheets.

At 716, method 700 includes cutting each of the first, second, and thirdthermoplastic sheets from the (final) roll to form the MEA. The MEA maysubsequently be added to an MEA stack, such as at 610 of method 600 (asdescribed in detail above with reference to FIG. 6 ).

Referring now to FIG. 8 , a schematic diagram 800 depicting an exemplaryroll-to-roll processing configuration 802 for assembling an MEA stack804 is shown. In an exemplary embodiment, within the roll-to-rollprocessing configuration 802, each MEA 806 of the MEA stack 804 may beindividually assembled via roll-to-roll processing of raw materialsheets 810 fed from respective rolls 808, which may be welded 814 to oneanother via thermal welders 812 at adherence regions thereof (e.g.,predetermined welding areas for hermetic sealing or structural integrityof the MEA 806 as finally formed) optionally following appropriateplacement of one or more electrode components between pairs of the rawmaterial sheets 810 to be welded 814, cut 816 once all of the rawmaterial sheets 810 have been welded 814 to corresponding raw materialsheets 810, and aligned and stacked with other MEAs 806 of the MEA stack804. In some examples, the raw material sheets 810 may be composed ofmaterials which may retain structural integrity upon being subjected totypical thermal welding temperatures, such as extruded thermoplasticsheets or frames, conductive sheets, and membrane sheets. In this way,manufacturing of the MEA stack 804 may be more efficient, accurate, andflexible than molding-based electrode assembly manufacturing processes,due to simple manufacturing design, reproducible high-precisiontolerances, and facile component replacement provided by the exemplaryroll-to-roll processing configuration 802.

In some examples, following assembly via the exemplary roll-to-rollprocessing configuration 802, the MEA stack 804 may be implemented in aredox flow battery system. In an exemplary embodiment, the redox flowbattery system may be the redox flow battery system 10 of FIG. 1 andeach MEA 806 of the MEA stack 804 may be configured as the MEA 204 ofFIGS. 2A-4B and 5A-5C. Additionally or alternatively, one or more stepsof method 600 of FIG. 6 or method 700 of FIG. 7 may be performed via theexemplary roll-to-roll processing configuration 802. For example, 602 ofmethod 600 of FIG. 6 and/or each of 702, 704, 706, 708, 710, 712, 714,and 716 of method 700 of FIG. 7 may be performed via the exemplaryroll-to-roll processing configuration 802. Accordingly, the exemplaryroll-to-roll processing configuration 802 may be considered withreference to the embodiments of FIGS. 1-7 (though it will be understoodthat similar roll-to-roll processing configurations may be employed tothe aforementioned or other embodiments without departing from the scopeof the present disclosure). It will further be appreciated that relativenumbers of the MEAs 806, the rolls 808, the raw material sheets 810, andthe thermal welders 812 depicted in the schematic diagram 800 of FIG. 8are exemplary and that greater or fewer MEAs 806 may be included in theMEA stack 804 and greater or fewer rolls 808, raw material sheets 810,and thermal welders 812 may be independently included in the exemplaryroll-to-roll processing configuration 802 without departing from thescope of the present disclosure.

In this way, an electrode assembly for a redox flow battery system maybe formed via roll-to-roll processing of extruded thermoplastics. In oneexample, sheets of the extruded thermoplastics may be adhered (e.g.,welded) to one another, as well as to other components of the electrodeassembly (e.g., a membrane sheet and conductive sheets), along a seriesof rolls to form various compartments (wherein further components may bepositioned during processing). Once formed, the electrode assembly maybe cut and added to a stack of electrode assemblies of likeconfiguration. One exemplary technical result of forming the electrodeassembly via roll-to-roll processing is that tolerance stacking may bedecreased relative to molding-based processes for electrode assemblymanufacturing (not only because roll-to-roll processing avoidsreplication of aberrations in a given mold, but also becauseroll-to-roll processing may be readily and cheaply adjusted to furtherreduce tolerance stacking). As such, expensive, specialized, andrelatively large molding tools and handling equipment may be obviated infavor of a roller setup.

Once formed, electrode assemblies may be attached to one another viainterlocking electrolyte distribution inserts located at respectiveelectrolyte ports of the electrode assemblies. The interlockingelectrolyte distribution inserts may be hermetically sealed onceinterlocked, forming common fluid manifolds through which electrolytemay be distributed throughout the electrode assemblies. During operationof the redox flow battery system, the extruded thermoplastics may expandor inflate responsive to increasing fluid pressure of circulatingelectrolyte. Accordingly, one exemplary technical result of employingthe extruded thermoplastics in manufacturing the electrode assembly isthat greater flexibility may be realized in forming the redox flowbattery system (e.g., housing or storage of the electrode assemblies maybe varied, testing probes may be positioned between electrode assemblieswhen deflated, dynamic flow restriction of the circulating electrolyte,etc.). Further, modularity of the electrode assemblies may permit facilereplacement of degraded electrode assemblies, as well as coupling ofadditional cells (e.g., electrolyte rebalancing cells) to the commonfluid manifolds via the interlocking electrolyte distribution inserts.

In one example, an electrode assembly for a redox flow battery, theelectrode assembly comprising: an inflatable housing, the inflatablehousing at least partially enclosing an internal volume, the internalvolume comprising negative and positive electrode compartments; anegative electrode spacer positioned in the negative electrodecompartment; and a positive electrode positioned in the positiveelectrode compartment, wherein the inflatable housing inflatesresponsive to applied internal pressure to increase the internal volumeof the electrode assembly during operation of the redox flow battery. Afirst example of the electrode assembly further includes wherein theinflatable housing comprises upper, middle, and lower extrudedthermoplastic frames, wherein the lower extruded thermoplastic frame isadhered to the middle extruded thermoplastic frame to form the negativeelectrode compartment, and wherein the upper extruded thermoplasticframe is adhered to the middle extruded thermoplastic frame to form thepositive electrode compartment. A second example of the electrodeassembly, optionally including the first example of the electrodeassembly, further comprises a first carbon fiber sheet adhered to thelower extruded thermoplastic frame; a membrane sheet adhered to themiddle extruded thermoplastic frame; and a second carbon fiber sheetadhered to the upper extruded thermoplastic frame, wherein the negativeelectrode spacer is positioned between the first carbon fiber sheet andthe membrane sheet, and wherein the positive electrode is positionedbetween the second carbon fiber sheet and the membrane sheet. A thirdexample of the electrode assembly, optionally including one or more ofthe first and second examples of the electrode assembly, furtherincludes wherein the upper, middle, and lower extruded thermoplasticframes are aligned to form a plurality of electrolyte ports, theplurality of electrolyte ports comprising negative electrolyte ports andpositive electrolyte ports, wherein the negative electrolyte ports arefluidically coupled to the negative electrode compartment, wherein thepositive electrolyte ports are fluidically coupled to the positiveelectrode compartment, and wherein the internal volume is hermeticallysealed during inflation of the inflatable housing excepting at theplurality of electrolyte ports. A fourth example of the electrodeassembly, optionally including one or more of the first through thirdexamples of the electrode assembly, further comprises electrolytedistribution inserts respectively circumscribing the plurality ofelectrolyte ports. A fifth example of the electrode assembly, optionallyincluding one or more of the first through fourth examples of theelectrode assembly, further includes wherein the negative electrolyteports are fluidically coupled to the negative electrode compartment viarespective negative electrolyte passages formed between the upperextruded thermoplastic frame and the middle extruded thermoplasticframe, and wherein the positive electrolyte ports are fluidicallycoupled to the positive electrode compartment via respective positiveelectrolyte passages formed between the lower extruded thermoplasticframe and the middle extruded thermoplastic frame. A sixth example ofthe electrode assembly, optionally including one or more of the firstthrough fifth examples of the electrode assembly, further includeswherein the negative electrolyte passages are sealed by welding of theupper extruded thermoplastic frame to the middle extruded thermoplasticframe, and wherein the positive electrolyte passages are sealed bywelding of the lower extruded thermoplastic frame to the middle extrudedthermoplastic frame. A seventh example of the electrode assembly,optionally including one or more of the first through sixth examples ofthe electrode assembly, further includes wherein the negative electrodespacer is formed from a non-conductive mesh, and wherein the positiveelectrode is formed from carbon felt. An eighth example of the electrodeassembly, optionally including one or more of the first through seventhexamples of the electrode assembly, further includes wherein no adhesiveis included between components of the electrode assembly.

In another example, a redox flow battery system, comprising: a pluralityof membrane electrode assemblies, each of the plurality of membraneelectrode assemblies comprising: an expandable thermoplastic housinghaving carbon fiber sheets welded to opposite sides thereof; a feltelectrode housed within the expandable thermoplastic housing; a meshelectrode spacer housed within the expandable thermoplastic housing; anda membrane sheet welded to an interior surface of the expandablethermoplastic housing, the membrane sheet being positioned between thefelt electrode and the mesh electrode spacer. A first example of theredox flow battery system further includes wherein electrolyte ports ofeach of the plurality of membrane electrode assemblies are aligned toform electrolyte manifolds fluidically coupling each of the plurality ofmembrane electrode assemblies to one another. A second example of theredox flow battery system, optionally including the first example of theredox flow battery system, further includes wherein each of theelectrolyte ports includes a circumferential electrolyte distributioninsert, and wherein respective circumferential electrolyte distributioninserts of pairs of adjacent membrane electrode assemblies of theplurality of membrane electrode assemblies mechanically interlock withone another. A third example of the redox flow battery system,optionally including one or more of the first and second examples of theredox flow battery system, further includes wherein the plurality ofmembrane electrode assemblies is arranged in a sequential stack, andwherein respective carbon fiber sheets of pairs of adjacent membraneelectrode assemblies of the plurality of membrane electrode assembliesare in face-sharing contact with one another. A fourth example of theredox flow battery system, optionally including one or more of the firstthrough third examples of the redox flow battery system, furthercomprises one or more rebalancing cell assemblies, wherein the one ormore rebalancing cell assemblies is arranged in the sequential stackwith the plurality of membrane electrode assemblies, and wherein the oneor more rebalancing cell assemblies is fluidically coupled with theplurality of membrane electrode assemblies. A fifth example of the redoxflow battery system, optionally including one or more of the firstthrough fourth examples of the redox flow battery system, furtherincludes wherein the redox flow battery system is an all-iron hybridredox flow battery system.

In yet another example, a method, comprising: sequentially stackinglayers to form each membrane electrode assembly of a stack of membraneelectrode assemblies via roll-to-roll processing; and operating thestack of membrane electrode assemblies as a redox flow battery whilepumping positive and negative electrolytes into the stack of membraneelectrode assemblies through respective fluid manifolds, the fluidmanifolds fluidically coupling each membrane electrode assembly of thestack of membrane electrode assemblies to one another. A first exampleof the method further includes wherein sequentially stacking the layersto form each membrane electrode assembly of the stack of membraneelectrode assemblies via roll-to-roll processing includes, for eachrespective membrane electrode assembly of the stack of membraneelectrode assemblies: welding a first conductive sheet to a firstthermoplastic sheet; positioning a negative electrode spacer on thefirst conductive sheet; welding a second thermoplastic sheet to thefirst thermoplastic sheet; welding a membrane sheet to the secondthermoplastic sheet; positioning a positive electrode on the membranesheet; welding a third thermoplastic sheet to the second thermoplasticsheet; and welding a second conductive sheet to the third thermoplasticsheet; and thereafter cutting the first, second, and third thermoplasticsheets from a roll to form the respective membrane electrode assembly. Asecond example of the method, optionally including the first example ofthe method, further includes wherein an electrical contact resistancebetween adjacent membrane electrode assemblies of the stack of membraneelectrode assemblies is lower while the stack of membrane electrodeassemblies is being operated as the redox flow battery than while thestack of membrane electrode assemblies is not being operated as theredox flow battery, wherein the stack of membrane electrode assembliesis expanded to a first volume via a first fluid pressure while the stackof membrane electrode assemblies is being operated as the redox flowbattery, wherein the stack of membrane electrode assemblies iscontracted to a second volume via a second fluid pressure while thestack of membrane electrode assemblies is not being operated as theredox flow battery, wherein the first volume is greater than the secondvolume, and wherein the first fluid pressure is greater than the secondfluid pressure. A third example of the method, optionally including oneor more of the first and second examples of the method, furthercomprises electrically coupling testing probes to at least one membraneassembly of the stack of membrane electrode assemblies while the stackof membrane electrode assemblies is not being operated as the redox flowbattery; conducting voltage testing of the at least one membraneelectrode assembly of the stack of membrane electrode assemblies via thetesting probes; and responsive to identifying one or more degradedmembrane electrode assemblies of the stack of membrane electrodeassemblies during the voltage testing, replacing the one or moredegraded membrane electrode assemblies. A fourth example of the method,optionally including one or more of the first through third examples ofthe method, further includes wherein each respective membrane electrodeassembly of the stack of membrane electrode assemblies includeschanneled electrolyte distribution inserts fluidically coupling aninterior of the respective membrane electrode assembly to the fluidmanifolds, and wherein the positive and negative electrolytes areadmitted into the interior of each membrane electrode assembly of thestack of membrane electrode assemblies from the fluid manifolds viachannels of the channeled electrolyte distribution inserts.

FIGS. 2A-5C show example configurations with relative positioning of thevarious components. If shown directly contacting each other, or directlycoupled, then such elements may be referred to as directly contacting ordirectly coupled, respectively, at least in one example. Similarly,elements shown contiguous or adjacent to one another may be contiguousor adjacent to each other, respectively, at least in one example. As anexample, components laying in face-sharing contact with each other maybe referred to as in face-sharing contact. As another example, elementspositioned apart from each other with only a space there-between and noother components may be referred to as such, in at least one example. Asyet another example, elements shown above/below one another, at oppositesides to one another, or to the left/right of one another may bereferred to as such, relative to one another. Further, as shown in thefigures, a topmost element or point of element may be referred to as a“top” of the component and a bottommost element or point of the elementmay be referred to as a “bottom” of the component, in at least oneexample. As used herein, top/bottom, upper/lower, above/below, may berelative to a vertical axis of the figures and used to describepositioning of elements of the figures relative to one another. As such,elements shown above other elements are positioned vertically above theother elements, in one example. As yet another example, shapes of theelements depicted within the figures may be referred to as having thoseshapes (e.g., such as being circular, straight, planar, curved, rounded,chamfered, angled, or the like). Further, elements shown intersectingone another may be referred to as intersecting elements or intersectingone another, in at least one example. Further still, an element shownwithin another element or shown outside of another element may bereferred as such, in one example. FIGS. 2A-5C are drawn approximately toscale, although other dimensions or relative dimensions may be used.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method, comprising: sequentially stacking layers to form eachmembrane electrode assembly of a stack of membrane electrode assembliesvia roll-to-roll processing; and operating the stack of membraneelectrode assemblies as a redox flow battery while pumping positive andnegative electrolytes into the stack of membrane electrode assembliesthrough respective fluid manifolds, the fluid manifolds fluidicallycoupling each membrane electrode assembly of the stack of membraneelectrode assemblies to one another.
 2. The method of claim 1, whereinsequentially stacking the layers to form each membrane electrodeassembly of the stack of membrane electrode assemblies via roll-to-rollprocessing includes, for each respective membrane electrode assembly ofthe stack of membrane electrode assemblies, includes: welding a firstconductive sheet to a first thermoplastic sheet; positioning a negativeelectrode spacer on the first conductive sheet; welding a secondthermoplastic sheet to the first thermoplastic sheet; welding a membranesheet to the second thermoplastic sheet; positioning a positiveelectrode on the membrane sheet; welding a third thermoplastic sheet tothe second thermoplastic sheet; and welding a second conductive sheet tothe third thermoplastic sheet; and thereafter cutting the first, second,and third thermoplastic sheets from a roll of each respectivethermoplastic sheet to form the respective membrane electrode assembly.3. The method of claim 1, wherein an electrical contact resistancebetween adjacent membrane electrode assemblies of the stack of membraneelectrode assemblies is lower while the stack of membrane electrodeassemblies is being operated as the redox flow battery than while thestack of membrane electrode assemblies is not being operated as theredox flow battery, wherein the stack of membrane electrode assembliesis expanded to a first volume via a first fluid pressure while the stackof membrane electrode assemblies is being operated as the redox flowbattery, wherein the stack of membrane electrode assemblies iscontracted to a second volume via a second fluid pressure while thestack of membrane electrode assemblies is not being operated as theredox flow battery, wherein the first volume is greater than the secondvolume, and wherein the first fluid pressure is greater than the secondfluid pressure.
 4. The method of claim 3, further comprising:electrically coupling testing probes to at least one membrane electrodeassembly of the stack of membrane electrode assemblies while the stackof membrane electrode assemblies is not being operated as the redox flowbattery; conducting voltage testing of the at least one membraneelectrode assembly of the stack of membrane electrode assemblies via thetesting probes; and responsive to identifying one or more degradedmembrane electrode assemblies of the stack of membrane electrodeassemblies during the voltage testing, replacing the one or moredegraded membrane electrode assemblies.
 5. The method of claim 1,wherein each respective membrane electrode assembly of the stack ofmembrane electrode assemblies includes channeled electrolytedistribution inserts fluidically coupling an interior of the respectivemembrane electrode assembly to the fluid manifolds, and wherein thepositive and negative electrolytes are admitted into the interior ofeach membrane electrode assembly of the stack of membrane electrodeassemblies from the fluid manifolds via channels of the channeledelectrolyte distribution inserts.
 6. A method, comprising: sequentiallystacking and adhering layers to form a membrane electrode assembly viaroll-to-roll processing; adding the membrane electrode assembly to amembrane electrode assembly stack; and operating the membrane electrodeassembly stack as a redox flow battery while pumping positive andnegative electrolytes into the membrane electrode assembly stack throughrespective fluid manifolds, wherein pumping positive and negativeelectrolytes inflates the membrane electrode assembly stack.
 7. Themethod of claim 6, wherein adding the membrane electrode assemblyincludes fluidically coupling an interior of the membrane electrodeassembly to the membrane electrode assembly stack via a channeledelectrolyte distribution insert.
 8. The method of claim 7, wherein therespective fluid manifolds are formed by interlocked channeledelectrolyte distribution inserts.
 9. The method of claim 6, furthercomprising after forming the membrane electrode assembly conducting apneumatic pressure test, including diagnosing a presence of a leak inseals of the membrane electrode assembly.
 10. The method of claim 6,wherein sequentially stacking and adhering layers to form the membraneelectrode assembly includes forming a membrane electrode assemblyhousing by adhering extruded thermoplastic sheets or frames, adheringconductive sheets to an exterior of the membrane electrode assemblyhousing, and adhering a membrane sheet to an interior surface of themembrane electrode assembly housing.
 11. The method of claim 10, whereinthe membrane sheet bisects an internal volume of the membrane electrodeassembly into positive and negative electrode compartments whilepermitting ionic conduction therebetween.
 12. The method of claim 6,wherein adhering includes thermally welding.
 13. The method of claim 6,wherein adhering does not include applying adhesive.
 14. The method ofclaim 6, wherein pumping positive and negative electrolytes includespumping positive and negative electrolytes comprised of iron ions.
 15. Amethod, comprising: feeding a first raw material sheet and a second rawmaterial sheet from a respective roll; placing one or more electrodecomponents between the first raw material sheet and the second rawmaterial sheet; and thermally welding the first raw material sheet tothe second raw material sheet, wherein the first and second raw materialsheets are included in a membrane electrode assembly of a membraneelectrode assembly stack configured to expand when operating as a redoxflow battery.
 16. The method of claim 15, wherein the one or moreelectrode components is a negative electrode spacer or a positiveelectrode.
 17. The method of claim 15, wherein the first raw materialsheet and the second raw material sheet are composed of materials whichmaintain structural integrity when thermally welded.
 18. The method ofclaim 15, wherein thermally welding includes thermally welding atadherence regions configured for hermetic sealing or structuralintegrity of the membrane electrode assembly.
 19. The method of claim15, wherein the first raw material sheet and the second raw materialsheet are one of extruded thermoplastic sheets, conductive sheets, ormembrane sheets.
 20. The method of claim 15, further comprising cuttingthe first raw material sheet and the second raw material sheet to formthe membrane electrode assembly.